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

I was using requirejs and picturefill, i was getting an intermittent problem where images were not always being loaded.  It was down to me loading picturefill deferred which i shouldn't really have been doing.

Took me a while to realise where i was going wrong, in the end i found this issue https://github.com/scottjehl/picturefill/issues/35 which explained the problem really well. I thought if we added a note to the readme it might help wayward souls like myself onto the correct path of enlightenment.