Update readme (#4090)

Author: PJUllrich

README.md

@@ -193,6 +193,7 @@ $ mix test
 
 Running all JavaScript tests:
 ```bash
+$ npm install
 $ npm run setup
 $ npm run test
 ```
@@ -220,4 +221,11 @@ $ npm run cover
 $ npm run cover:report
 ```
 
+Format the files:
+
+```bash
+$ mix format
+$ npm run js:format
+```
+
 JS contributions are very welcome, but please do not include an updated `priv/static/phoenix_live_view.js` in pull requests. The maintainers will update it as part of the release process.