document the .reviewboardrc usage for REVIEWBOARD_URL

Review Request #371 — Created May 7, 2008 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
For people that don't use svn or git, having a .reviewboardrc file containing REVIEWBOARD_URL is also sufficient.
I added this to the documentation of post-review
this review was posted with the changed file (also note that I only added documentation of existing functionality)