Write module and function docs

[finnhodgkin]

There should be solid documentation for the module as a whole as well and all non-private functions:

login_url/0, login_url/1 and github_auth/1

[ZooeyMiller]

I forgot to add the info about adding the module to our the deps in mix.exs but I'll get on that tomorrow :+1:

EDIT: I'll add this in when we've set up the stuff for Hex, because I think that will be a prerequisite for getting this sorted.