Add CONTRIBUTING.md.

i.e.

![https://github-images.s3.amazonaws.com/skitch/issues-20120913-162539.jpg](https://github-images.s3.amazonaws.com/skitch/issues-20120913-162539.jpg)

This may help improve the overall quality of issues and reduce number of duplicates & trivial/non-bug bug reports.

Alternatively the content could live in the wiki, and just be linked from `CONTRIBUTING.md`.

This is just a rough draft, happy to revise/start over.