Created a CONTRIBUTING.pod for GitHub

[NReilingh]

Mentioned this idea in #349 -- thought I'd write it up in case you like it.

This file is a convenience for new contributors perusing the repository on GitHub.com. Basically it just functions as a signpost to the resources that people should be accessing (like docs/hacking.pod).

The presence of this file activates some GitHub features documented at Setting guidelines for repository contributors - GitHub Docs. This includes links on the "New pull request" form, and the https://github.com/bestpractical/rt/contribute route.

This functionality can also be activated by locating the file at .github/CONTRIBUTING.pod, but I think it's more convenient to have it visible at the repository root.

I tested the GitHub rendering of this Pod markup at my fork.