Adding CONTRIBUTING and CONTRIBUTORS files

Adding temp page for CONTRIBUTING.md (not sure what it should have in it yet, or what we want to move from README.md into there).

Also created an initial CONTRIBUTORS.md file with all the current contributors. You can update this by calling `$ npm run contributors`. Output can be tweaked as far as you can customize `$ git shortlog -s -n` output. I make no guarantees how it will work on Windows or other platforms, but "works on my machine".

Closes #590