Document category names to be avoided. Closes #622.
Document category names to be avoided. Closes #622.

PalmBeachPost/panda,PalmBeachPost/panda,datadesk/panda,PalmBeachPost/panda,NUKnightLab/panda,ibrahimcesar/panda,NUKnightLab/panda,pandaproject/panda,ibrahimcesar/panda,NUKnightLab/panda,PalmBeachPost/panda,pandaproject/panda,ibrahimcesar/panda,datadesk/panda,ibrahimcesar/panda,NUKnightLab/panda,pandaproject/panda,newsapps/panda,newsapps/panda,datadesk/panda,newsapps/panda,ibrahimcesar/panda,datadesk/panda,PalmBeachPost/panda,datadesk/panda,pandaproject/panda,newsapps/panda,pandaproject/panda