Add radio receiver and radio transmitter examples.

See more
https://devzone.nordicsemi.com/documentation/nrf51/4.4.1/html/group__nrf__dev__led__radio__rx__example.html
https://devzone.nordicsemi.com/documentation/nrf51/4.4.1/html/group__nrf__dev__led__radio__tx__example.html

I was unable to send '0' and '1' via UART, so the transmitter example just sends '0' all the time.
Hopefully, that will be fixed soon.