Add workflow for making Pull Requests

[souravsingh]

This would be good for new contributors who wish to contribute to the project.Perhaps add a CONTRIBUTING file.

[aktech]

This document could be cleaned up and added.
https://github.com/pydsa/pydsa/wiki/Contributing-to-PyDSA

[CuriousLearner]

Also I would suggest to add a proper work-flow for working on different PRs at same time and general instructions for git. That would surely help new contributors!

[souravsingh]

Nice Idea @CuriousLearner