CONTRIBUTING.md: Replace AstroPy links with GitHub's Flow guide

GitHub just deployed some guides [1], so we can reverence them (with
their pretty graphics and accessible UI) instead of the AstroPy docs.

GitHub's guide is pretty, but it doesn't go into lots of technical
detail (e.g. how do I make a commit or open a pull request?), so we
still need a link to a Git tutorial.  I've replaced the old AstroPy
link with one to our own novice lesson, since we might as well eat our
own dog food.

[1]: https://github.com/blog/1769-announcing-guides
Merge pull request #279 from wking/github-workflow-guide

CONTRIBUTING.md: Replace AstroPy links with GitHub's Flow guide