update README, expand 'before the workshop', add links to 'further resources'

**Update README with instructions for contributing**

Everyone knows how to fork and make pull requests so this is a trivial addition. But might as well spell it out so people know changes are welcome.

**Expand 'Before the workshop' resources**

Added items mentioned in Lillie's Pivotal Tacker. Also added a link to my "What is Ruby on Rails?" primer as "Advance Reading." I wrote it after I coached my first RailsBridge workshop because I saw some people needed more background and context. Feel free to remove it if you think it is inappropriate (I won't be offended).

**Add links to 'Further Resources' page**

Added a link to my "What is Ruby on Rails?" under a "Background" heading.

Added a link to my "Rails Tutorials" recommendations page under "Want to learn more Rails?".

I think these links are useful (but of course I'm biased).