Add docs on how to change number system

[nirvdrum]

The README doesn't indicate that there are different number systems or how to switch between them. I stumbled upon it while looking at the TypeScript definitions. I'd be interested in trying out the 64 char system in order to expand the search space while not blowing up memory, but I don't see a straightforward way of changing the number system.

[Venryx]

Not sure if the API is the same, but there is a fork of this repo called "@wewatch/lexorank" that shows how to do this: https://github.com/wewatch/lexorank#customize

EDIT: It looks like that configuration API is specific to the fork: wewatch/lexorank@b8742f1