Add documentation on how to contribute to the project

[codersquid]

Add documentation on how to contribute that you can point to as a starting point for new people to read on how to contribute. For example, when a new person shows up in irc, you'd point them to this.

As for where the documentation could live -- github treats CONTRIBUTING.md as a first class citizen, so you could put the documentation in that, and then also put it in the README as a link. Or, you could document it on the wiki and have a link to that in CONTRIBUTING and the README.