Rewrite and restructure the README, and add CONTRIBUTING

A description and justification of the changes are available in the two commits.

Some friends and I were diving into Meteor last week, and found Meteorite. We really had trouble browsing and understanding the documentation, so I wanted to make it better.

This is my first open-source contribution. I hope I've done everything alright. I've tried to follow best practices; branching, squashing, etc.

Looking forward to your feedback!