Summary:
- Break some README contents out into separate docs files.
- Add a short sample to the README.
- Add rationale (mostly copied from internal wiki).
- Check in generated quickref.md.
Pull Request resolved: https://github.com/facebookincubator/fbjni/pull/22

Reviewed By: passy

Differential Revision: D18168116

Pulled By: dreiss

fbshipit-source-id: 2ed4c8b7447f7fbc89d2433ce67dd2977e80c9f4