1. Updates the README.md to have a heading with the word "contribute", long
   time OSS contributors (or maybe I just do this) skim docs quickly and need
   a large call out to exactly what they are looking for.
2. The best resource I found was a developer guide that wasn't even linked in
   CONTRIBUTING.md, which seems like a shame so I made it right at the top!

Signed-off-by: Jess Frazelle <me@jessfraz.com>