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: Michael Rodin <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/admin-guide/README.rst b/Documentation/admin-guide/README.rst
index 02caa7e..15ea785 100644
--- a/Documentation/admin-guide/README.rst
+++ b/Documentation/admin-guide/README.rst
@@ -1,3 +1,5 @@
+.. _readme:
+
 Linux kernel release 4.x <http://kernel.org/>
 =============================================