RBE Configuration using Buildfarm

[Al Sutton]

I've seen, a few times, folk bring up the topic of RBE based AOSP builds, so, having recently got things up and running with Buildfarm,  I've put some documentation up at buildfarm/contrib/aosp at add_aosp_build_client_details · alsutton/buildfarm and submitted a PR to get this merged into the Buildfarm main project.

Feedback would be appreciated..

Al.