Help: Add release note for Objective-C/C++ language support

Help/release/dev/objective-c-cxx.rst

@@ -0,0 +1,9 @@
+Objective C/C++
+---------------
+
+* CMake learned to support the Objective C (``OBJC``) and Objective C++
+  (``OBJCXX``) languages.  They may be enabled via the :command:`project`
+  and :command:`enable_language` commands.  When ``OBJC`` or ``OBJCXX``
+  is enabled, source files with the ``.m`` or ``.mm``, respectively,
+  will be compiled as Objective C or C++.  Otherwise they will be treated
+  as plain C++ sources as they were before.