Document RC voting guidelines in ref guide

Document all necessary and suggested steps to vote on a release.

There are only a handful necessary checks a PMC member must do on every
release, and all of them relate to packaging, LICENSE and NOTICE files, and
license auditing, which can be accomplished by running the RAT tool, by
attempting to compile from source (unit tests optional), and through manual
inspection of LICENSE and NOTICE files in the source distribution and
embedded in a sample of the binaries. This entire process should take you
less than 15 minutes, from my experience. This is the baseline.

Any individual PMCer may opt to do more than the baseline, but it is
optional. Personally I would also read the compatibility report, and then
run the unit test suite in the background and come back to it when finished
to complete the voting task. In my opinion now that is the baseline tasks
any HBase PMC voter should take. Beyond that, at least for my releases, you
can read the vote email to find the additional functional and performance
checks I might have done and factor that in to your voting confidence. You
can also run them yourselves, but is totally optional.