gpio: document interaction with other subsystems

Now I am very fed up with people reinventing kernel wheels in
userspace "just because they can" (read, sysfs). Put in a angry
blurb in sysfs doc and put in a new file with pointers to other
subsystem drivers utilizing GPIOs.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/Documentation/gpio/00-INDEX b/Documentation/gpio/00-INDEX
index 1de43ae..179beb2 100644
--- a/Documentation/gpio/00-INDEX
+++ b/Documentation/gpio/00-INDEX
@@ -6,6 +6,9 @@
 	- How to obtain and use GPIOs in a driver
 driver.txt
 	- How to write a GPIO driver
+drivers-on-gpio.txt:
+	- Drivers in other subsystems that can use GPIO to provide more
+	  complex functionality.
 board.txt
 	- How to assign GPIOs to a consumer device and a function
 sysfs.txt