[Device GPIO] Add pages describing how to test (with real hardware)

[jpwhitemn]

Per Kamakura planning meeting (Nov 21), each device service should provide a HOWTOTEST.md page that describe how to check the base functionality of the device service with real hardware. These tests should be run manually before each release.

Each device service should also provide a WORKSWITH.md page that lists the actual hardware (make, model, web site link, etc.) that the device service has been tested with and known to work with. Provide enough details on the device/sensor so that an adopter would know where to go to get the same device.

[lenny-goodell]

moving this to docs repo now that the service documentation is there.