chore(build): document FAST_BUILD environment variable flag
chore(build): document FAST_BUILD environment variable flag

related to CAM-6214

xlinur/camunda-bpm-webapp,TitanUser/camunda-bpm-webapp,TitanUser/camunda-bpm-webapp,xlinur/camunda-bpm-webapp,xlinur/camunda-bpm-webapp,TitanUser/camunda-bpm-webapp