Fix missing server extension activation in README (#1157)

Author: fcollonval

README.md

@@ -211,6 +211,8 @@ pip install -e ".[dev]"
 pre-commit install
 # Link your development version of the extension with JupyterLab
 jupyter labextension develop . --overwrite
+# Server extension must be manually installed in develop mode
+jupyter server extension enable jupyterlab_git
 # Rebuild extension Typescript source after making changes
 jlpm run build
 ```