Add valve example

Used for gardena valves with 9V power supply.

examples/osd/arduino-valve/README.md

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+Arduino valve
+=============
+
+This is an example how to open/close a Gardena Water-Valve (the ones
+that use a +9V block battery for power).
+Note that we used the L293D Dual H-Bridge circuit which needs +5V for
+power supply (while the Merkur-Board uses 3.3V) but it can use 3.3V
+inputs directly from the Merkur-Board without a level shifter.  So it's
+probably a good idea to use some other H-Bridge circuit that can also
+use 3.3V. An advantage of the L293D might be that it can be used for two
+valves simultaneously (with the second, currently unused H-Bridge).
+
+We provide the schematics of our breadboard-design in
+valve-breadboard.png -- as noted above it's a good idea to use a
+different H-Bridge circuit.
+
+Also note that one of the pins we use in the software for signals to the
+H-Bridge (D4) is also used for the LED on the Merkurboard. The LED is
+*on* when the H-Bridge is *off*. This was nice for testing the
+breadboard circuit but should be changed for a real deployment to save
+power of the battery. It is also probably a good idea to turn off the
+power-supply of the H-Bridge with an additional MOSFET depending on the
+power consumption of the H-Bridge you're going to use.