Documentation: Added missing `labextension link` command

README.md

@@ -14,7 +14,8 @@ jupyter labextension install gathering-tools
 For a development install (requires npm version 4 or later), do the following in the repository directory:
 
 ```bash
-npm install
+npm install                   # download dependencies
+jupyter labextension link .   # install this package in Jupyter Lab
 jlpm run watch                # automatically recompile sources
 jupyter lab --watch           # launch Jupyter Lab, automatically re-load extension
 ```