MAINTAINERS: Add ubifs-authentication.rst to UBIFS

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200905204326.1378339-2-j.neuschaefer@gmx.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

MAINTAINERS

@@ -17699,6 +17699,7 @@ S:	Supported
 W:	http://www.linux-mtd.infradead.org/doc/ubifs.html
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/rw/ubifs.git next
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/rw/ubifs.git fixes
+F:	Documentation/filesystems/ubifs-authentication.rst
 F:	Documentation/filesystems/ubifs.rst
 F:	fs/ubifs/