Add a contributing guide

[leouieda]

If we have a file called CONTRIBUTING.md in the repository root, Github will know what it is and will mention it when people open issues or pull requests. It should contain basic information about cloning the repository, building the code, writing a test, writing an example for the docs, etc. We can start with something derived from the GMT/Python guide and expand it over time.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[leouieda]

Started in #332