Meaning of a Shakespeare line

In Act 5, scene iii of Richard III, Richard is trying to rally his troops for the Battle of Bosworth. Someone brings him a note that says

“Jockey of Norfolk, be not so bold,
For Dickon thy master is bought and sold.”

Richard then says that this is the work of an enemy spy. I can’t understand what the note means, and neither of my two annotated editions explains it. Help!

http://www.r3.org/bosworth/legends.html

Page down for the note on the tent.

The meaning would be roughly, “John of Norfolk, beware, King Richard is betrayed.”
Jock/Jockey and Dickon/Dick were northern England and Scots variants of the names John and Richard. Compare “bought and sold” to modern slang “owned”/“pwned”

“Your cause is betrayed. Save yourself while you can.”