This issue continues what has been started in pre-commit/pre-commit-hooks#217

So @asottile, what do you want me to merge (by adapting what I wrote of course) in pre-commit/pre-commit/CONTRIBUTING.md?

Proposed changes:

1. Create tests/README.md
   • to move testing related stuff in CONTRIBUTING.md, and make a link from CONTRIBUTING.md to tests/README.md. I think contributing is not necessarily developing feature/fixing bug;
   • or at least to link to CONTRIBUTING.md.
2. Get rid of the supported Python versions, or at least point to tox.ini
3. Keep the Git configuration interference
   • it is a temporary workaround waiting the bug to be fixed;
   • we can mention to open an issue if this kind of problem occurs.