MAINTAINERS: add entry for Documentation/admin-guide/reporting-issues.rst

Thorsten will keep an eye on the new document about reporting issues
(aka bugs).

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/c8d55ec74d104b90fdb9c155bca3b407e8480fb3.1617113469.git.linux@leemhuis.info


Signed-off-by: Jonathan Corbet <corbet@lwn.net>