docs: guide for upgrading dependencies (#243)

docs/README.md

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 - [Concept - URL Rewriting](./concepts/url-rewriting.md)
 - [Guide - Mocking REST API Calls](./guides/mocking-rest-calls.md)
 
+### Maintenance
+
+- [Guide - Upgrading Dependencies](./guides/upgrading-pwa.md)
+
 ## Operations
 
 ### Setup

docs/guides/upgrading-pwa.md

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+<!--
+kb_guide
+kb_pwa
+kb_everyone
+kb_sync_latest_only
+-->
+
+# Upgrading Dependencies
+
+This guide gives a short overview on how to proceed with upgrading PWA dependencies.
+
+## Customer Projects
+
+In most cases, customer projects should never do upgrades of dependencies themselves.
+Whenever a new PWA version is released, the project should consume it completely, following the recommended [importing instructions](./customizations.md#import-changes-from-new-release).
+Especially the [`package-lock.json`](./customizations.md#dependencies) should be reset to Intershop's version and then (if customized) rewritten using `npm install`.
+
+## PWA Development
+
+The following section should only be abided by Intershop PWA developers and projects that want to disconnect from Intershop's PWA update stream.
+
+### Recommendations for the Process of Updating
+
+Always use `ng update` for upgrading packages, as it also collects and executes possible source code transformers that do some part of automatic code migration.
+We would also recommend updating with `ng update -C` which creates (temporary) commits for each individual update.
+This way the process can be tracked easier while doing the update.
+
+After each package update run some form of code check to verify code consistency.
+Full checks can be run with `npm run check`.
+You can also run specific subparts of it for verification.
+
+Do not upgrade too many packages at once as you can easily lose track of the process and have to start anew.
+
+### 0. Before the Update
+
+Check if all third-party libraries for the PWA are compatible with the new version.
+This is especially important while doing a major Angular version upgrade.
+
+### 1. Updating Angular Dependencies
+
+If a new major Angular version should be consumed, follow the steps in the [official update guide](https://update.angular.io) for advanced app complexity.
+
+`ng update` gives an overview of available Angular-specific package updates:
+
+```text
+$ ng update
+...
+Found 99 dependencies.
+    We analyzed your package.json, there are some packages to update:
+
+      Name                               Version                  Command to update
+     --------------------------------------------------------------------------------
+      @angular/cli                       9.1.4 -> 9.1.6           ng update @angular/cli
+      @angular/core                      9.1.6 -> 9.1.7           ng update @angular/core
+      @nguniversal/express-engine        9.1.0 -> 9.1.1           ng update @nguniversal/express-engine
+```
+
+### 2. Updating Third-Party Project Dependencies
+
+After updating the Angular-specific dependencies, update project libraries (everything you use on the PWA).
+
+`npm outdated` gives an overview of all upgradable packages in `package.json`:
+
+```text
+$ npm outdated --long
+...
+@types/jest                          25.2.1    25.2.3   25.2.3  intershop-pwa  devDependencies  https://github.com/DefinitelyTyped/DefinitelyTyped#readme
+@types/node                        12.12.38  12.12.39   14.0.1  intershop-pwa  devDependencies  https://github.com/DefinitelyTyped/DefinitelyTyped#readme
+angulartics2                          9.0.0     9.0.1    9.0.1  intershop-pwa  dependencies     https://angulartics.github.io/angulartics2/
+bootstrap                             4.4.1     4.5.0    4.5.0  intershop-pwa  dependencies     https://getbootstrap.com/
+...
+```
+
+Perform updates with `ng update` as well.
+
+:warning: `@types/node` should always remain on the LTS version.
+You can upgrade to specific versions with `ng update @types/node@12`.
+
+### 3. Update Project Utilities for Testing, Reporting and Linting
+
+Usually utility dependencies can be upgraded individually as they do not interfere with production code directly.
+In most cases you can also postpone these updates for a longer time.
+
+### 4. Cleanup Dependencies
+
+Sometimes build processes and libraries require you to install peer dependencies yourself.
+However, this can change after version upgrades and those dependencies become obsolete.
+You can check with `npm ls <package-name>` if a package is used or required.
+
+After removing potential candidates check `npm install` for peer dependency requirements and also run the responsible process to assure continued functionality.
+
+### 5. Update Utilities for Formatting
+
+Especially `prettier` upgrades and applying formatting should be done in individual commits (one for the upgrade and a single one only applying the formatting), so customer projects can ignore the second one and run the formatting independently.
+
+### 6. Refactoring and Deprecations
+
+Now would be a good time to apply optional code refactoring and replace deprecated code artifacts in individual commits.
+
+### 7. Restructuring the Update and Documentation
+
+For customer projects it is most important that the process of upgrading is transparent and applicable.
+The commits leading to this point should be partially merged and restructured, so that they are importable via cherry-picking.
+
+One recommendation would be to combine all manipulations to `package-lock.json` in one single commit if no major refactorings or library replacement took place while upgrading.
+Further commits should be focused on descriptive commit messages, so the upgrading project can follow them easily.
+At the end code re-formatting and optional refactorings should reside in individual commits.
+
+:warning: Every commit along the way must be consistent. `npm run check` must be runnable without errors, so the customer project can use it to assure consistency.
+
+Add documentation with migration instructions to the [migration guide](./migrations.md).
+
+## Security Vulnerabilities
+
+Sometimes `npm install` reports security vulnerabilities:
+
+```text
+$ npm install
+...
+found 11 vulnerabilities (8 low, 3 high)
+  run `npm audit fix` to fix them, or `npm audit` for details
+```
+
+Do not pay too much attention to this.
+By doing a complete install on the project, all dependencies (production and development) are audited, but only production dependencies will end up in the production code.
+You should, however, pay attention to the production audit:
+
+```text
+$ npm audit --production
+
+                       === npm audit security report ===
+
+found 0 vulnerabilities
+ in 237 scanned packages
+```
+
+If a package (in most cases a transitive dependency) poses a security risk, most likely an update of the consuming package is already available.
+Go ahead and update the package following the steps above.
+The output of `npm audit` will also give useful information about packages.
+
+### :warning: DO NOT RUN `npm audit fix`!
+
+This would upgrade versions of transitive dependencies directly in `package-lock.json` and occasionally the affected utilities will stop working.