ACPI: Update MAINTAINERS file to include Documentation/acpi

Documentation/acpi contains all of the Docunmentation for the
Linux/ACPI subsystem.  Adds this to the MAINTAINERS file.
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

MAINTAINERS

@@ -242,6 +242,7 @@ F:	drivers/acpi/
 F:	drivers/pnp/pnpacpi/
 F:	include/linux/acpi.h
 F:	include/acpi/
+F:	Documentation/acpi
 
 ACPI FAN DRIVER
 M:	Zhang Rui <rui.zhang@intel.com>