Document all `new Octokit({ throttle })` options

[gr2m]

Add a section to the README for usage with GitHub Enterprise, too, as rate limit options are configurable in GHE

[stephen-dunn]

@gr2m - any update for adding github Enterprise support?

[payne911]

The documentation is very much lacking.

I changed from using onAbuseLimit to using onSecondaryRateLimit (https://github.com/useful-forks/useful-forks.github.io/commit/739eba52f9ba40d94e56218d43da75ebedcfe02d), but the doc leaves me wondering about so many things related to the return values and passed parameters of both onRateLimit and onSecondaryRateLimit...