Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Other Goblin creatures you control attack each combat if able.
At the beginning of combat on your turn, create a 1/1 red Goblin creature token with haste.
Whenever Goblin Rabblemaster attacks, it gets +1/+0 until end of turn for each other attacking Goblin.
7/18/2014 Although Goblin Rabblemaster doesn’t force itself to attack, if you control two of them, they’ll force each other to attack if able.
7/18/2014 If, during your declare attackers step, a creature that must attack if able is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under your control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having a creature attack, you’re not forced to pay that cost, so it doesn’t have to attack in that case either.
7/18/2014 The number of attacking Goblins is counted as the last ability resolves, and the bonus is locked in at that time.