Update README.md

Add note describing iOS app store safety.
Sorry to pile on further, but can we clarify that this is temporary issue and not inherent to Carthage itself? Maybe something like:

> :warning: Frameworks built using Carthage will currently be rejected in iOS App Store submissions. This is a [known issue] that should be fixed in a future release.

I think we should also:

 1. Move this into the section about getting started, rather than qualifying our big mission statement with it.
 1. Offer a workaround—adding projects directly? stripping the simulator bits?