Update README with note about yarn install

Author: rossta

README.md

@@ -108,6 +108,12 @@ Optional: To fix ["unmet peer dependency" warnings](https://github.com/rails/web
 yarn upgrade
 ```
 
+When `package.json` and/or `yarn.lock` changes, such as when pulling down changes to your local environemnt in a team settings, be sure to keep your NPM packages up-to-date:
+
+```bash
+yarn install
+```
+
 ### Usage
 
 Once installed, you can start writing modern ES6-flavored JavaScript apps right away: