Add Carthage build folder and explaination

Author: Nate West

Swift.gitignore

@@ -24,3 +24,10 @@ DerivedData
 # http://guides.cocoapods.org/using/using-cocoapods.html#should-i-ignore-the-pods-directory-in-source-control
 #
 # Pods/
+
+# Carthage
+#
+# Add this line if you want to avoid checking in source code from Carthage dependencies.
+# Carthage/Checkouts
+
+Carthage/Build