Rewrite and expand docs on auto-JS scripts

This adds a note about automatically re-running JavaScript tests.
I've moved and rewritten the old content about re-building JS code
to match the new style.

README.md

@@ -55,29 +55,29 @@ make run-flask
 
 Then visit [localhost:6012](http://localhost:6012).
 
+Any Python code changes you make should be picked up automatically in development. If you're developing JavaScript code, run `npm run watch` to achieve the same.
+
 ## To test the application
 
 ```
 # install dependencies, etc.
 make bootstrap
 
+# run all the tests
 make test
+
+# continuously run js tests
+npm run test-watch
 ```
 
+To run a specific JavaScript test, you'll need to copy the full command from `package.json`.
+
 ## Common tasks
 
 ### Updating application dependencies
 
 `requirements.txt` is generated from the `requirements.in` in order to pin versions of all nested dependencies. If `requirements.in` has been changed, run `make freeze-requirements` to regenerate it.
 
-### Automatically rebuild the frontend assets
-
-If you want the front end assets to re-compile on changes, leave this running in a separate terminal from the app
-
-```shell
-    npm run watch
-```
-
 ## Further docs
 
 - [Working with static assets](docs/static-assets.md)