Fix https://codeforphilly.github.io/chime/CONTRIBUTING.html to reflect new PR process

[josh-bluewater]

Doc currently shows older info requesting contributers to

• "Base all work on the latest master branch available in Code for Philly's fork of the chime repository.
• Submit your work in the form of a pull request against that same branch."

[themightychris]

This was recently fixed in #100 and will be reflected in the published docs after we merge #95