MAINTAINERS: tty: add serial docs directory

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -10098,6 +10098,7 @@ M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 M:	Jiri Slaby <jslaby@suse.cz>
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
+F:	Documentation/serial/
 F:	drivers/tty/
 F:	drivers/tty/serial/serial_core.c
 F:	include/linux/serial_core.h