Change README.md so it's clear setup instructions are for development
Change README.md so it's clear setup instructions are for development

Instructions could confuse people who just want to download the zip
and use the files, instead of developing with the .ts files.

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