add docs on feature flags to the README
add docs on feature flags to the README

chef/supermarket,robbkidd/supermarket,jzohrab/supermarket,nellshamrell/supermarket,chef/supermarket,juliandunn/supermarket,chef/supermarket,nellshamrell/supermarket,leftathome/supermarket,tas50/supermarket,dpnl87/supermarket,rafaelmagu/supermarket,leftathome/supermarket,rafaelmagu/supermarket,chef/supermarket,jzohrab/supermarket,dpnl87/supermarket,chef/supermarket,robbkidd/supermarket,dpnl87/supermarket,rafaelmagu/supermarket,tas50/supermarket,nellshamrell/supermarket,juliandunn/supermarket,juliandunn/supermarket,leftathome/supermarket,rafaelmagu/supermarket,tas50/supermarket,juliandunn/supermarket,jzohrab/supermarket,dpnl87/supermarket,tas50/supermarket,nellshamrell/supermarket,robbkidd/supermarket,robbkidd/supermarket,leftathome/supermarket,jzohrab/supermarket