added some documentation about how to create a new authentication provider
added some documentation about how to create a new authentication provider

CarsonF/Silex,fabpot/Silex,beealone/Silex,romainneutron/Silex,cgsmith/Silex,mmauri04/Silex,jdreesen/Silex,cesarmarinhorj/Silex,Xephi/Silex,SanderKnape/Silex,peterkokot/Silex,Tobion/Silex,naderman/Silex,romainneutron/Silex,peterkokot/Silex,plotek/Silex,mmauri04/Silex,Xephi/Silex,mathroc/Silex,GromNaN/Silex,Tobion/Silex,arduanov/Silex,lokhman/Silex,GromNaN/Silex,Incognito/silex-fork,fhinkel/Silex,fabpot/Silex,JPry/Silex,tcx1000/Silex,tcx1000/Silex,RayRutjes/Silex,akucherevskiy/developex_test,GromNaN/Silex,SpacePossum/Silex,CarsonF/Silex,cgsmith/Silex,peterkokot/Silex,SpacePossum/Silex,beealone/Silex,gencer/Silex,akucherevskiy/developex_test,mTorres/Silex,arduanov/Silex,CarsonF/Silex,SanderKnape/Silex,JPry/Silex,lyrixx/Silex,mathroc/Silex,jdreesen/Silex,lokhman/Silex,lyrixx/Silex,Tobion/Silex,romainneutron/Silex,fhinkel/Silex,cesarmarinhorj/Silex,plotek/Silex,naderman/Silex,akucherevskiy/developex_test,Incognito/silex-fork,naderman/Silex,lyrixx/Silex,silexphp/Silex,mTorres/Silex,RayRutjes/Silex