Update docs

CONTRIBUTING.md

@@ -193,7 +193,7 @@ to build the changes and then refresh your browser to see the changes.
 To have the system build after each source file change, run:
 
 ```bash
-npm run watch
+jupyter lab --dev-mode --watch
 ```
 
 and refresh the browser.

docs/extensions_dev.md

@@ -90,6 +90,15 @@ packages are included in `jupyter labextension list`.
 You can also use `jupyter labextension install <path>`, but that will
 only copy the current contents of the source folder.
 
+When using linked packages, you can run the command
+
+```
+jupyter lab --watch
+```
+
+This will cause the application to incrementally rebuild when one of the
+linked packages changes.
+
 Note that the application is built against **released** versions of the
 core JupyterLab extensions.  If your extension depends on JupyterLab
 packages, it should be compatible with the dependencies in the