Contributing.md

In light of 9c7210d &
https://github.com/artursapek/mondrian/pull/24#issuecomment-33337824
Rename to all caps
Shorter header
Rework headers and rephrase commandment
Stop using CLI js-yaml

This eliminates the global js-yaml dependency
Update README to clarify easy setup
Fix localhost link
Move things around between README and CONTRIBUTING
Simplify headers
Sanity print after CSS compilation
Lowercase continuations of sentence