[ci skip] docs: warn about errant use of devDependencies (#1880)

docs/yarn.md

@@ -1,6 +1,6 @@
 # Yarn
 
-Webpacker by default uses `yarn` as package manager for node modules
+Webpacker by default uses `yarn` as a package manager for `node_modules`
 
 
 ## Add a new npm module
@@ -10,3 +10,14 @@ To add any new JS module you can use `yarn`:
 ```bash
 yarn add bootstrap material-ui
 ```
+
+## Add an npm module to `devDependencies`
+To add a new JS module that will only be available to local development:
+
+```bash
+yarn add --dev browser-sync
+```
+
+Be careful not to add any build or app related JS modules in this fashion. Adding JS modules to `devDependencies` [will block them from being installed in **any** production environment](https://yarnpkg.com/lang/en/docs/cli/install/#toc-yarn-install-production-true-false).
+
+Docs from JS modules may instruct you to use `--dev` or `devDependencies`, but this is generally under the assumption that you are using a `node.js` workflow.