Remove warnings about Xcode 5.0.1 in the documentation [skip ci]

Subliminal is now compatible with Xcode 5.0.1 and this warnings are
outdated. Let’s remove them from README.md.
Document issue about swiping table view cell [skip ci]

In prior versions of iOS, UITableViewCell instances used a discrete
swipe gesture recognizer to implement the ‘swipe-to-delete’ behavior
and show the deletion confirmation button.

With iOS 7 the internal view hierarchy of a cell has been modified:
a scroll view has been inserted between the UITableViewCell and the
content view. It is responsible for tracking the swiping gesture and
set the inset for the whole content view.

Since Subliminal cannot drag scroll view in the iOS 7 Simulator for
the time being, swiping a table view cell doesn’t work either. Let’s
add this as a known issue in README.md.