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.
Remove this content from the `cmake-developer(7)` manual because it
is relevant only to developers working on CMake itself. Move it to
a guide in the developer documentation.
Teach Sphinx and our own install rules to skip a `Help/dev` directory.
This will give us a place to put developer-only documentation that
should not be included in the user-facing documentation. Add a
placeholder README.
