Update readme for watch mode (#535)

* Update readme for watch mode

* Updated README per review feedback

* Update README.md

Co-Authored-By: Frédéric Collonval <[email protected]>

Co-authored-by: Frédéric Collonval <[email protected]>

Author: mlucool

README.md

@@ -120,6 +120,16 @@ jlpm run build
 jupyter lab build
 ```
 
+Or start jupyter in watch mode:
+```bash
+jupyter lab --watch
+```
+And in a separate session, start this project in watch mode:
+```bash
+jlpm run watch
+```
+Now every change will be built locally then bundled into Jupyter Lab. Refresh the page after any save to see your change (note: you'll need to wait for webpack to finish, which can take 10s+ at times).
+
 To execute the tests
 
 ```bash