Added a note in the readme regarding deferred loading of picturefill.
Added a note in the readme regarding deferred loading of picturefill.

footiealf/picturefill,chrismcandrew/picturefill,albell/picturefill,yufi113/picturefill,samiadelgado/picturefill,scottjehl/picturefill,rockmandew/picturefill,footiealf/picturefill,demateu/picturefill,guardian/picturefill,nucliweb/picturefill,iancrowther/picturefill,rockmandew/picturefill,yufi113/picturefill,scottjehl/picturefill,mcanthony/picturefill,mcanthony/picturefill,yubin-huang/picturefill,iancrowther/picturefill,purposeindustries/picturefill,purposeindustries/picturefill,guardian/picturefill,jansepar/picturefill,yubin-huang/picturefill,albell/picturefill,patrickkettner/picturefill,ddeveloperr/picturefill,demateu/picturefill,samiadelgado/picturefill,ddeveloperr/picturefill,chrismcandrew/picturefill,patrickkettner/picturefill