Update CONTRIBUTING.md
https://github.com/casesandberg/react-color/blob/53335f5b26a480495de703b53981d73f5acf78e6/.github/CONTRIBUTING.md

I assume it is outdated, since there's no gulpfile?

To be honest I have no idea how to start with react-color locally. I cloned the repo, did a `npm install` and checked the `package.json` for scripts. I then ran `npm run docs` which gave me

> ERROR in ./~/react-addons-shallow-compare/index.js
> Module not found: Error: Cannot resolve 'file' or 'directory' /home/alex/things/src/react-color/node_modules/react/lib/shallowCompare in /home/alex/things/src/react-color/node_modules/react-addons-shallow-compare
>  @ ./~/react-addons-shallow-compare/index.js 1:17-52

It would be great if the CONTRIBUTING.md could be updated to contain guidelines for how to contribute. Thanks!
