Add more policy documents

[joeyparrish]

GitHub recommends adding certain "community health" files to your repositories, with specific names established by convention. #47 adds SECURITY.md and CODE_OF_CONDUCT.md to all repos. There are a few more I think we should add:

• CONTRIBUTING.md (many repos have a version of this already, but they are not in sync or much customized to shaka-project; see also Google boilerplate at https://github.com/google/new-project/blob/master/docs/contributing.md)
• SUPPORT.md (none of our repos have this yet)

The workflow in #47 will keep these in sync across repos. They merely have to be written, added to the policies/ folder, then reviewed & merged.