Help/dev: Add a CMake Experimental Features Guide

Initialize it with placeholder content.  This document will serve to
contain documentation for experimental features that are under
development and not yet included in official documentation.

Help/dev/README.rst

@@ -37,9 +37,11 @@ CMake developer documentation is provided by the following documents:
 
 * The `CMake Source Code Guide`_.
 * The `CMake Documentation Guide`_.
+* The `CMake Experimental Features Guide`_.
 
 .. _`CMake Source Code Guide`: source.rst
 .. _`CMake Documentation Guide`: documentation.rst
+.. _`CMake Experimental Features Guide`: experimental.rst
 
 Maintainer Documentation
 ========================

Help/dev/experimental.rst

@@ -0,0 +1,10 @@
+CMake Experimental Features Guide
+*********************************
+
+The following is a guide to CMake experimental features that are
+under development and not yet included in official documentation.
+See documentation on `CMake Development`_ for more information.
+
+.. _`CMake Development`: README.rst
+
+No experimental features are under development in this version of CMake.