[readme] - respond to justin's comments. also add API change warning.
[readme] - respond to justin's comments. also add API change warning.

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