MAINTAINERS: Add the docs-next git tree to the maintainer entry

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

MAINTAINERS

@@ -3207,6 +3207,7 @@ F:	Documentation/
 X:	Documentation/ABI/
 X:	Documentation/devicetree/
 X:	Documentation/[a-z][a-z]_[A-Z][A-Z]/
+T:	git git://git.lwn.net/linux-2.6.git docs-next
 
 DOUBLETALK DRIVER
 M:	"James R. Van Zandt" <jrv@vanzandt.mv.com>