Add 'Contributing' section to README.md

Add a section to the README that describes how to
* get the code (with git and github)
* make changes (self-evident) and test them (via devtools), and
* make a pull request (with git and github).

You may or may not wish to include the first and last item, but the middle item is probably sensible to have in a README (or separate DEVELOPMENT file), as it explains how to work on knitr / run its test suite?