Documentation: add link to stable release candidate tree

There is also stable release candidate tree. Mention it, however with a
warning that the tree is for testing purposes.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220314113329.485372-5-bagasdotme@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/process/stable-kernel-rules.rst

@@ -172,6 +172,15 @@ Trees
 
 	https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
 
+ - The release candidate of all stable kernel versions can be found at:
+
+        https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git/
+
+   .. warning::
+      The -stable-rc tree is a snapshot in time of the stable-queue tree and
+      will change frequently, hence will be rebased often. It should only be
+      used for testing purposes (e.g. to be consumed by CI systems).
+
 
 Review committee
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