CONTRIBUTING.md: Rename from 'CONTRIBUTE.md'

To match GitHub's conventions [1].  With this change, folks will get
links to the contributing docs when they open a new issue or
pull-request using GitHub's web UI.

[1]: https://github.com/blog/1184-contributing-guidelines
CONTRIBUTING.md: Point closed-source hub issues to support-index@

To give @shin- a break from redirecting people ;).
Merge pull request #579 from wking/contributing

CONTRIBUTE.md → CONTRIBUTING.md and a support-index@ reference