Documentation: add link to stable release candidate tree

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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>