Update README to note how to just run WordPress tests
Update README to note how to just run WordPress tests

See @gedex: https://github.com/x-team/WordPress-Coding-Standards/pull/27#issuecomment-15592466

mattyrob/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,xwp/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,lkraav/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,mediarain/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,borkweb/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,borkweb/WordPress-Coding-Standards,borkweb/WordPress-Coding-Standards,lkraav/WordPress-Coding-Standards,borkweb/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,xwp/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,mattyrob/WordPress-Coding-Standards,xwp/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,xwp/WordPress-Coding-Standards,mediarain/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards