Add contributor documentation

[willingc]

At a sprint and an attendee would like to contribute to the project, let's make this easier by providing a contributor page with how to install a development environment and run the tests. cc @jzf2101

[willingc]

General instructions for setting up a development environment:

1. Fork and clone repo
2. Use conda (from Anaconda or miniconda) to install dependencies

create conda -n qtdev python=3
conda activate qtdev
conda install pyqt
pip install -e .

[wmvanvliet]

To symlink the qtconsole repository dir into your site-packages:

python setup.py develop

(you don't want a regular install of qtconsole, because you want any changes you make in the repository dir to take effect immediately while developing)

[willingc]

Either @wmvanvliet suggestion for immediate changes or pip install -e . when run after changes are made to the source code (essentially creates a development install ref) may be used. If you are creating many changes the first approach is preferred.

To run tests:

nosetests