DOC: Difficulty Novice tag in contributing section doesn't exist

[mvinoba]

The Difficulty Novice tag recommended in the main README.md Contributing section doesn't seem to exist anymore. I think the right recommendation should be the good first issue tag, just like it's in the Where to start? section in the docs for contributing to the documentation. Same applies to /doc/source/contributing.rst.

I can work on this and open a PR if the issue is confirmed.

[WillAyd]

Seems logical - go for it!