Rewrite and restructure the README, and add CONTRIBUTING

Here are direct links to the files to see what they look like with GitHub's rendering:

* [README.md](https://github.com/zenmunki/meteorite/blob/19f3cbfc45982869009f59433ccd22426324cbee/README.md)
* [CONTRIBUTING.md](https://github.com/zenmunki/meteorite/blob/19f3cbfc45982869009f59433ccd22426324cbee/CONTRIBUTING.md)

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!