Document `github:` and custom URL schemes

[AkihiroSuda]

[AkihiroSuda]

@jandubois Do you have any draft of the documentation?

[jandubois]

@jandubois Do you have any draft of the documentation?

I'll try to write it up tonight.

[jandubois]

I'm working on the github: URL scheme docs right now. So far I'm putting it into website/content/en/docs/templates/github.md. Let me know if you want it somewhere else.

Not sure if I will get around to the custom URL plugin support today as well. It is a little more obscure anyways.

And this is not part of the release bits, so we can still improve it over the coming days.