update readme with info about options
update readme with info about options

nbrustein/omniauth,ngpestelos/omniauth,dougal/omniauth,intridea/omniauth,twalpole/omniauth,thomasmillergb/Year2-ruby-software-hut,yui-knk/omniauth,ayunav/omniauth,sideci-sample/sideci-sample-omniauth,mihar/omniauth-timekiwi,likeberi/omniauth,thomasmillergb/Year2-ruby-software-hut,omniauth/omniauth,poznyakandrey/omniauth