Add test instructions - #217

Just realized there is https://github.com/pre-commit/pre-commit/blob/master/CONTRIBUTING.md

FML

Maybe some parts can be merged? Especially the parts where the existing Git configuration interfere with the test runner. I've been quite annoyed by this problem.

While I think this is all correct and useful information, a lot of it is duplicated with the general tox documentation and CONTRIBUTING in pre-commit/pre-commit.

Note also that if you find your git configuration conflicting with the tests, we consider it a bug, that should be fixed and would welcome changes to improve the testsuite or hooks themselves to be more resistant to alternate git configurations :)

I'm also not sure enumerating the supported python versions is necessary or useful as that information is already in setup.py and tox.ini (two standard places to search for this information).

We'll handle this in pre-commit/pre-commit#564