Extend jupyterlab extension build instructions in README

Author: jonmmease

README.md

@@ -66,3 +66,9 @@ Then install the JupyterLab extension in development mode:
 ```
 $ jupyter labextension link extensions/jupyterlab
 ```
+
+For release, build the JupyterLab extension to bundle with the Python package:
+
+```
+$ python setup.py build_js
+```