project: Add DESCRIPTION parameter

It is quite often the project description has used in a real world software.
Examples include:

* part of a help screen of the application
* builtin resources (`*.rc` files, data for "About" dialog of a GUI app, & etc)
* most generators for CPack can use it
* it could be used by documentary software (Doxygen, Sphinx) which is usually
  integrated to CMake based projects via `add_custom_target()`

Now `project()` call learned an optional `DESCRIPTION` parameter with a
short string describing a project. Being specified, it would set the
`PROJECT_DESCRIPTION` variable which could be used in `configure_file()`
or whatever user wants.  Also `PROJECT_DESCRIPTION` is a default value
for `CPACK_PACKAGE_DESCRIPTION_SUMMARY`.

Help/variable/CMAKE_PROJECT_DESCRIPTION.rst

@@ -0,0 +1,7 @@
+CMAKE_PROJECT_DESCRIPTION
+-------------------------
+
+The description of the current project.
+
+This specifies description of the current project from the closest inherited
+:command:`project` command.

Help/variable/PROJECT_DESCRIPTION.rst

@@ -0,0 +1,6 @@
+PROJECT_DESCRIPTION
+-------------------
+
+Short project description given to the project command.
+
+This is the description given to the most recent :command:`project` command.