Added Image Optimization section

as a subsection of Performance...
Not sure if the order is chosen correctly — change it, if you find a better position.

Off topic: The README is getting a bit difficult to navigate. It would be great to have some accordion or [nestable](http://dbushell.com/2012/06/17/nestable-jquery-plugin/) or at least a table of content (with anchors to the sections). I don't know if this is possible in GitHub though.