Updated README for extra keys setting
Updated README for extra keys setting

artburkart/voluptuous,tuukkamustonen/voluptuous,natlownes/voluptuous,alecthomas/voluptuous,alecthomas/voluptuous,divanovGH/voluptuous,SimplyKnownAsG/voluptuous,tuukkamustonen/voluptuous,SimplyKnownAsG/voluptuous,jmwri/voluptuous