CONTRIBUTING.md: point to external docs on GitHub

CONTRIBUTING.md: point to external docs on GitHub

This translates CONTRIBUTING (from the `contributing` branch) into
Markdown using a GitHub URL for the link.  Merging this into your
project will set it up to use GitHub's CONTRIBUTING infrastructure,
and will be used to notify users creating issues and pull requests
[1].

GitHub's blob URL syntax is [2]:

  https://github.com/<user>/<project>/blob/<commit-SHA-1>/Path/To/File

Like CONTRIBUTING, CONTRIBUTING.md is also released under the CC0
Universal license (see the `license` branch for full text).

[1]: https://github.com/blog/1184-contributing-guidelines
[2]: https://help.github.com/articles/how-do-i-get-a-permanent-link-from-file-view-to-permanent-blob-url

Signed-off-by: W. Trevor King <wking@tremily.us>