Add Contributing.rst

[raymond-devries]

I would like to contribute but the process of getting the development environment set up is not very clear. Adding a contributing.rst with instructions on how to set up a development environment, run pytest, and standard practice for how pull requests should be submitted would be extremely helpful.

[paulocheque]

Thanks for the help:

For now, you can try:

1. Clone the repository
2. make prepare
3. make os_deps (please convert the OS code to the system you are using)
4. make deps
5. make test