No docs other than the README (either user or developer)

[mpacer]

Currently nbviewer looks to be largely undocumented (outside of the README) for either user or developer docs.

I'm not sure if this was a choice, and I don't know as much about documentation for javascript applications, but this seems (at best) suboptimal.

[minrk]

suboptimal is generous :)

nbviewer's grown well beyond its original scope, and we should probably put together more thorough docs. In particular, it was not originally intended to be deployed anywhere other than nbviewer.jupyter.org, so covering the many ways it can now deployed didn't originally make sense.

Now that it's somewhat organically become a 'real' service that others can deploy, we should take a run at covering how this can be done, describing the 'providers' notion, etc.