Contributor guide

[nabriis]

I noticed that there is no contributor guide in the docs. A contributor guide is very useful for first-time contributors. It is also useful for providing any (novice or experienced) contributors with specifics related to this project, like how to run tests!

Since I just went through the process I could take a stab at it. Might need a bit of guidance for how to build/run tests.

I really like the short summary in the NumPy contributor guide.

Some suggested points to focus on:

• [ ] Issue guideline. Create issue before PR.
• [ ] Step-by-step guide on creating a PR. (fork, clone fork, add remote, commit changes, run tests, push changes to fork, create PR)
• [ ] How to install dependencies for tests and run them
• [ ] Review process: How to request review.
• [ ] Labels: How to manage labels (if needed).
• [ ] Updating fork from remote.
• [ ] License considerations.