Help: Add notes for topic 'xcode-message-extension'

Help/release/dev/xcode-message-extension.rst

@@ -0,0 +1,7 @@
+xcode-message-extension
+-----------------------
+
+* New :prop_tgt:`XCODE_PRODUCT_TYPE` and :prop_tgt:`XCODE_EXPLICIT_FILE_TYPE`
+  target properties were created to tell the :generator:`Xcode` generator
+  to use custom values of the corresponding attributes for a target in the
+  generated Xcode project.