Add new slack_channel_created event to README. [ci skip]
Add new slack_channel_created event to README. [ci skip]

pallan/lita-slack,SCPR/lita-slack,clintkelly/lita-slack,librato/lita-slack,byroot/lita-slack,ajaleelp/lita-slack,erikogan/lita-slack,litaio/lita-slack,josacar/lita-slack,kenjij/lita-slack,jaisonerick/lita-slack