Include instructions for contributing new frameworks

[dohliam]

It should be much easier to include new frameworks now, since all that is required is to add the relevant information to frameworks.yml and the repo will update itself automatically. Step-by-step instructions on how to do this probably be helpful for new contributors though. These should be included in a CONTRIBUTING.md file and linked from the main readme.

This should include information about adding new frameworks vs. adding new collections of frameworks (or expanding an existing collection).