Documentation/admin-guide/README.rst: add a label for cross-referencing

Add a label to the top of the file to allow cross-referencing.
Currently it's not possible to cross-reference this file from
Documentation/process/howto.rst because of the missing label.
Signed-off-by: NMichael Rodin <michael-git@rodin.online>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/README.rst

+.. _readme:
+
 Linux kernel release 4.x <http://kernel.org/>
 =============================================