Adds linker flags suggestion to README.mdown

Hey @jverkoey, I found it necessary when adding my framework to a dependent project to add the -ObjC and -all_load linker flags (Xcode 4.5.2). I added this suggestion to the instructions along with a few more small changes.

This changeset also:

* Suggests removing the Copy Files build phase (as discussed in #32)
* Adds a screenshot for the optional linker flags (at `gfx/linker_flags.png` in my fork, so you may want to update the URL after merging)
* Also adds alt values for images