Better Docs for the Relay-4 Board

The C0135 "Relay-4" board has 4 relays (duh), 4 inputs, and is based on the STM8S103F3P6 (the lower spec STM8S003F3P6 would have done the job just as well).

I now updated the docs in the GitHub Wiki, and added some example Forth code that demonstrates how to use it as a heating / neutral / cooling temperature control (as if there wouldn't be enough Chinese STM8S003F3P6 based thermostats boards).

By the way, right now this board is available on AliExpress for just $7.67, which isn't a bad price for a "Nano PLC.