Update README to mention gitter per this interaction on IRC
Update README to mention gitter per this interaction on IRC

<fkchang> jeremyevans: it seems that https://gitter.im/opal/opal get's a lot
          more chat love than irc these days, I'd bring your tilt discussion
          there                                                         [15:43]
<jeremyevans> fkchang: Thanks                                           [15:48]
<jeremyevans> Maybe the README could use an update                      [15:49]

Ajedi32/opal,Mogztter/opal,opal/opal,wied03/opal,Ajedi32/opal,merongivian/opal,kachick/opal,kachick/opal,Flikofbluelight747/opal,wied03/opal,Mogztter/opal,suyesh/opal,Flikofbluelight747/opal,gabrielrios/opal,gausie/opal,castwide/opal,bbatsov/opal,kachick/opal,iliabylich/opal,castwide/opal,fazibear/opal,jannishuebl/opal,gabrielrios/opal,c42engineering/opal,fazibear/opal,kachick/opal,jannishuebl/opal,bbatsov/opal,iliabylich/opal,gausie/opal,castwide/opal,wied03/opal,merongivian/opal,opal/opal,Mogztter/opal,c42engineering/opal,opal/opal,jgaskins/opal,fazibear/opal,gausie/opal,c42engineering/opal,bbatsov/opal,jannishuebl/opal,iliabylich/opal,Ajedi32/opal,suyesh/opal,suyesh/opal,jgaskins/opal,Mogztter/opal,opal/opal,Flikofbluelight747/opal,merongivian/opal,jgaskins/opal,gabrielrios/opal