Document how to use Rand with WASM

[vks]

Currently, we don't document how Rand can be used with WASM. In particular:

• We don't mention WASM in the rand readme or documentation. I think we should at least suggest to consult getrandom's documentation, because some feature flags may be required (also see Error when using Shuffle in a wasm project #990).
• In the Rand book, we claim that getrandom does not use feature flags, which is confusing (and arguably wrong, because they are required for some WASM targets):

getrandom (and by extension rand_os) has limited support for no_std. It does not use feature flags, instead detecting requirements based on the target platform.

[burdges]

There exist wasm targets where getrandom should never work, which may warrant a comment somewhere.

[dhardy]

Sounds sensible, but I don't have the perspective to know what's most useful here.

[Spoonbender]

It would actually be nice to be able to use rand's getrandom feature in a wasm32-unknown-unknown environment with JS. How about adding a "JS" feature to rand, which will enable the JS feature of getrandom?

[newpavlov]

@Spoonbender
We had a similar feature in rand v0.7, but we intentionally have removed it in v0.8. The recommended way now is to enable the getrandom feature directly in application crates as described in the docs: https://docs.rs/getrandom/0.2.2/getrandom/#indirect-dependencies

[Spoonbender]

That makes a lot of sense actually, TYVM