[WIP] writing some docs

It's a bit copy-paste, but thankfully the build will propagate the xml-docs from interface to implementation, so less :poop: work all round

 - [ ] document `IGitHubClient`
 - [ ] document `IObservableGitHubClient`
 - [ ] ???
 
I'll break out the TODOs here into related tasks so that others can help out with the work (it's also breaking changes so softly, softly might be the way to go)