docs(readme): add setup instructions, and infor terraform version con…

…straint syntax

README.md

@@ -16,16 +16,71 @@ The purpose of the repo is to easily test custom config for various terraform
 and terragrunt [renovate
 managers](https://docs.renovatebot.com/modules/manager/) in one place.
 
+## Setup
+
+The easiest way to use Renovate is to use the GitHub app, see
+[here](https://docs.renovatebot.com/install-github-app/) for the deployment
+options.
+
+I opted to run it locally to give me more control over when it runs whilst I
+experiment:
+
+```sh
+npm i -g renovate
+export RENOVATE_TOKEN=<my GitHub token>
+LOG_LEVEL=debug renovate --autodiscover --autodiscover-filter sajid-khan-js/renovate-test
+```
+
+It's also handy to run this after you make changes to your config
+(`renovate.json`) `renovate-config-validator` (comes with npm install of
+renovate)
+
 ## Structure
 
-- [infra](./infra) Terragrunt deployment files - Renovate will bump upstream
-  module versions
-  - `.terraform-version` and `.terragrunt-version` files are used by `tfenv` and
-      `tgenv` respectively - Renovate will bump the version numbers in these
-      files
-- [modules](./modules) Terraform modules - Renovate will bump provider and
-  terraform version pins
-- [renovate.json](./renovate.json) - Config file for renovate
+### [Infra](./infra)
+
+Terragrunt deployment files.
+
+Renovate will bump:
+
+- Upstream module versions e.g.
+  `github.com/cloudposse/terraform-null-label?ref=0.24.0`
+- `.terraform-version` and `.terragrunt-version` files are used by `tfenv` and
+  `tgenv` respectively - Renovate will bump the version numbers in these files
+  to the latest respective version
+
+### [Modules](./modules)
+
+Terraform modules/stacks.
+
+Renovate will bump:
+
+- Provider and terraform version pins in specific cases
+- Any upstream modules used in the stacks e.g. `github.com/cloudposse/terraform-null-label?ref=0.24.0`
+
+#### Terraform version constraint syntax
+
+:memo: Renovate is aware of the [different terraform version
+constraints](https://www.terraform.io/docs/language/expressions/version-constraints.html),
+meaning it doesn't simply bump versions but actually evaluates whether a bump is
+needed. Read more
+[here](https://docs.renovatebot.com/modules/manager/terraform/) and
+[here](https://docs.renovatebot.com/configuration-options/#rangestrategy)
+
+- `>= 2.0` will not be bumped since this pin allows any newer version that `2.0`
+      i.e. at runtime (when you running `terraform init`), terraform will pull
+      the latest provider version anyway
+- `~> 0.14.4` will be bumped, but since the pessimistic operator is used (`~>`)
+  it will use a version that satisfies this constraint e.g. if the latest
+  version is `1.0.4`, bumping to to `1.0.0` will still make the stack usable by
+  anyone running terraform version `1.0.X`
+
+### [Renovate config](./renovate.json)
+
+Config file for renovate.
+
+See [here](https://docs.renovatebot.com/configuration-options/) for
+configuration options
 
 ## Renovate in action