Add undocumented functions to README.md

[gdalle]

Hey there,
I was recently looking for an efficient way to enumerate permutations in-place, and I had to look in the source files to find nthperm!(a, k). Are there more super-useful but undocumented functions like this? Which ones should be added to the README?

[logankilpatrick]

Hey @gdalle if you want to tackle this and open a PR, happy to review it.

[gdalle]

I've started to collect hidden function names in my fork, but I was wondering if it would be time to add a Documenter.jl workflow, so that docstrings don't have to be duplicated inside the README?

[logankilpatrick]

Yes, that would be great to have. Would be awesome if you could tackle this but if not, I will get to it sometime in the next month.

[gdalle]

See #112