Add explanation of CryptoSwift target problem

Author: Tom Kemp

Cartfile

@@ -1,3 +1,8 @@
+# CryptoSwift versions 0.9 - 0.10 inclusive have iOS deployment
+# target "9.0" set in the Xcode project (but not in the Podspec).
+# PusherSwift has target "8.0" therefore the version in this
+# file can't match PusherSwift's Podspec's minimum CryptoSwift
+# version currently.
 github "krzyzanowskim/CryptoSwift" ~> 0.15.0
 github "ashleymills/Reachability.swift" == 4.3.0
 github "daltoniam/Starscream" == 3.0.6