[bug 1087280] Add feed categories help text.
[bug 1087280] Add feed categories help text.

chirilo/remo,tsmrachel/remo,abdullah2891/remo,johngian/remo,flamingspaz/remo,akatsoulas/remo,abdullah2891/remo,akatsoulas/remo,chirilo/remo,johngian/remo,akatsoulas/remo,mozilla/remo,flamingspaz/remo,tsmrachel/remo,mozilla/remo,johngian/remo,chirilo/remo,flamingspaz/remo,chirilo/remo,flamingspaz/remo,mozilla/remo,Mte90/remo,Mte90/remo,tsmrachel/remo,tsmrachel/remo,johngian/remo,abdullah2891/remo,abdullah2891/remo,mozilla/remo,Mte90/remo,akatsoulas/remo,Mte90/remo