Update README

Summary:
- Moved directory explanation list into a table. Easier on the eyes.
- Added contribution guidelines, including information regarding the CLA.
- Merged in [Pascal Borreli's typo fix](facebook#6) for "HTTPTransaction".
Closes https://github.com/facebook/proxygen/pull/10
Github Author: Adam McArthur <adam@adammcarthur.net>

{sync, type="child", parent="github", parentrepo="facebook/proxygen", parentprnum="10", parentprfbid="353515734828382"}

Test Plan: review

Reviewed By: viswanath@fb.com

Subscribers: doug, bmatheny

FB internal diff: D1666829

Signature: t1:1666829:1415382090:87ffd769d8e7215acde01969c0b13ec48fb135e1
Bump version to 1:0