Add a CONTRIBUTING file

[Ana06]

I think we should add a CONTRIBUTING file to collect some important information we now have in other documents. I information is usually in the CONTRIBUTING file in other project and it is where people expect it to be. In addition, it is used by GitHub to help guiding new contributors. For example, when someone opens a pull request or creates an issue, they will see a link to that file:

Reference: https://help.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors

I think this document should include the following information:

• How to contribute with issues, including a reference to the capa-rules repository and which issues belongs to every repo. This should also be linked from the issues template.
• How to write rules, linking current documentation and explaining the linter
• How to contribute with code, including how to set the project up (currently in different documents) and how to run the tests.

Something else?

[williballenthin]

i like the example used by atom here: https://github.com/atom/atom/blob/master/CONTRIBUTING.md