Added development installation guide

Author: kolibril13

README.md

@@ -41,6 +41,20 @@ conda install -c conda-forge nodejs
 jupyter labextension install @jupyter-widgets/jupyterlab-manager ipycanvas
 ```
 
+## For a development installation (requires npm)
+```bash
+$git clone https://github.com/martinRenou/ipycanvas
+$ cd ipycanvas
+$ pip install -e .
+$ jupyter nbextension install --py --symlink --sys-prefix ipycanvas
+$ jupyter nbextension enable --py --sys-prefix ipycanvas
+$ jupyter labextension install @jupyter-widgets/jupyterlab-manager
+$ jupyter labextension install js
+```
+When actively developing your extension, build Jupyter Lab with the command:
+```bash
+$ jupyter lab --watch
+```
 ## Examples
 
 ### Create John Conway's Game Of Life