Adding README for acceptance test directory

Adding a landing page doc file to help new users with running acceptance tests. Right now there are a lot of environment variables that are non-obvious to set up.
I'd add the following verbiage, as I've had some failures in the past where resources stuck around (e.g., when a server is updated, but doesn't complete within "go test"'s timeout period, you might occasionally find the errant server left over):

These tests should remove their work products automatically.  However, there may exist some specialized failure modes which we cannot foresee.  Always be vigilent and double-check test-related resources are suitably scrubbed away after you run these tests.