Help: Add notes for topic 'AppleClang-5.1-features'

Help/release/dev/AppleClang-5.1-features.rst

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+AppleClang-5.1-features
+-----------------------
+
+* The :manual:`Compile Features <cmake-compile-features(7)>` functionality
+  is now aware of features supported by Apple Clang 5.1 (``AppleClang``).