DOC: Update contributing for test_fast, fix doc Windows build
- Add `test_fast.bat` for Windows developers.
- Update `contributing.rst` to refer to `pytest-xdist` and reference fast testing scripts
- Update `contributing.rst` to delete references to `pep8radius`
- Fix `doc\make.py` to not spit out error message about `rm` on Windows by using `os.remove()`

No code changes!

minor comment, otherwise lgtm. thanks @Dr-Irv yeah I killed the appveyor/travis jobs (circleci does auto-killing itself :)

 @jorisvandenbossche ? @Dr-Irv Thanks!