Fix up contributing guide

[mfosterw]

The current contributing guide provides no real information and should be rewritten based on a template like this one, while of course also including the information added for #23.

[mfosterw]

Thanks to mine and others' work it no longer needs to be rewritten, but should be given a once-over to make sure everything is still consistent and up to date (I know at least a couple things are not).