update README.md to reference getdeps.py for build and tests

Summary:
Update the README.md to reflect that getdeps.py build and test is the method tested in CI and thus preferred.

Where getdeps.py manifests encode dependencies I've removed the manually specified dependences from the README for consistency (e.g. googletest version in README was outdated, manifest is correct and is tested in CI)

Also referenced main vs master and improved markdown header formatting a little

Reviewed By: meyering

Differential Revision: D33295900

fbshipit-source-id: caec3b3795be0b37bf1efdf12f4dbf23a37a023c