Add example on how to use datachannels standalone

[ValorZard]

I’m using Pion for a certain project of mine, and I only really care about the datachannels bits of the WebRTC api + I’m not having a web client. So, I’d like to see an example on how to use datachannels standalone

[jmonticelli]

https://github.com/pion/webrtc has a datachannel test if that is helpful: https://github.com/pion/webrtc/blob/master/datachannel_test.go