Contributor introduction document

Summary:
Orient potential contributors to stuff they should read first, the Facebook CLA,
how they can get started, and the general philosophy of the project.

Test Plan:
read the document

Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen, davidrecordon
Commenters: davidrecordon
CC: aran, epriestley, davidrecordon
Differential Revision: 208