acer-wmi - Add documentation

Add some initial documentation detailing what acer-wmi is, and how to use
it. Update the Kconfig entry with a reference to the documentation.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/Documentation/laptops/00-INDEX b/Documentation/laptops/00-INDEX
index dbe1759..729c2c0 100644
--- a/Documentation/laptops/00-INDEX
+++ b/Documentation/laptops/00-INDEX
@@ -1,5 +1,7 @@
 00-INDEX
 	- This file
+acer-wmi.txt
+	- information on the Acer Laptop WMI Extras driver.
 sony-laptop.txt
 	- Sony Notebook Control Driver (SNC) Readme.
 sonypi.txt