Unify Contributor Docs

Right now we have overlapping and sometimes contradictory docs on how to setup and build BEAM in four different places I've found:

README.md
CONTRIBUTING.md
https://cwiki.apache.org/confluence/display/BEAM/Contributor+FAQ
https://beam.apache.org/contribute/

We should probably pick one as the source of truth and rewrite the
other three to simply point to it. I propose putting all checkout,
build, test, commit, and push instructions in CONTRIBUTING.md in the
repo.