Documentation: Clarify that to use StableRNGs one must give a `rng` to every `rand()` call instead of setting Random.seed!()

[eirikbrandsaas]

Hi, thanks for this package. Very useful as my computational cluster doesn't update Julia frequently.

Issue:
I think many just set Random.seed!() and forget about it. Then for some reason they need to use StableRNGs, at which point the natural starting point is something like Random.seed!(StableRNG(seed::Integer)) which doesn't work. (e.g., https://discourse.julialang.org/t/random-seed-lts-vs-latest-stable/64370).

The usage section could maybe just have an extra subsection at the end of readme.md, along the lines of:

Moving from Random.seed!() to StableRNGs

If you in previous code used `Random.seed!() to get the same random numbers, you most now modify all calls that generate random numbers. For example, if your old code was:

Random.seed!(1)
rand()

It should be:

using StableRNGs
rng = StableRNG(1) # Note that we don't use `Random.seed!()` anymore as it is redundant.
rand(rng,)