add docs on deferred object

[desandro]

#15 does a lot of awesome. the README should have docs on how to used the jQuery Deferred object returned by imagesLoaded.

[darsain]

docs added, check it & close

[desandro]

Niiiiiiiice :)

Pro-tip: you can reference GitHub issues from a commit like so: #17 and GitHub will make a nice annotation. See desandro/3dtransforms#1 for an example

[fredleblanc]

It may be worth noting that the deferred additions (or, at least, use of deferred.notify()) ups the required jQuery version to v1.7.

[desandro]

@fredhq Good point, opening #18