Clean up the readme.
Clean up the readme.

Stop linking to the Google group.
Direct people to our Gitter for help.
Link to tutorials no longer claims to link to the docs.

gdseller/plottable,alyssaq/plottable,danmane/plottable,NextTuesday/plottable,danmane/plottable,softwords/plottable,softwords/plottable,palantir/plottable,palantir/plottable,jacqt/plottable,onaio/plottable,jacqt/plottable,gdseller/plottable,iobeam/plottable,RobertoMalatesta/plottable,RobertoMalatesta/plottable,RobertoMalatesta/plottable,alyssaq/plottable,onaio/plottable,danmane/plottable,alyssaq/plottable,softwords/plottable,iobeam/plottable,iobeam/plottable,jacqt/plottable,palantir/plottable,gdseller/plottable,onaio/plottable,palantir/plottable,NextTuesday/plottable,NextTuesday/plottable