Contributing - Add a section to README.md

[consideRatio]

I wanted to make an improvement, but it took quite a while until I managed to setup a working development environment. A contributing section like in Jupyterhubs repository would have helped me!

Perhaps something like this could be added to the readme?

Contributing

1. Fork
2. Clone and setup

git clone https://github.com/ipython-contrib/jupyter_contrib_nbextensions.git
cd jupyter_contrib_nbextensions
pip install --editable .
jupyter-contrib-nbextension install --symlink --sys-prefix

3. Develop
4. Pull request