Suggestion: update docs to indicate "async generators" support

[getify]

I found Highland while searching for stream library support for native JS async generators (like async function *main(..) { .. }) and thus async-iteration. It seems from looking at the code and previous commits that these are supported when passing to the _(..) stream constructor, but the docs don't really make that clear.

The docs mention support for "Iterator" (from normal generators), but not "AsyncIterator"... that is, where the .next(..) call returns Promise<IteratorResult>.

I would suggest including mention of this support, including code examples, to reduce future confusion.

[maxdumas]

Is this true? It makes sense that this would be the case, and either way I imagine it would be trivial to support. As a TypeScript user, it would be great if the Typescript bindings supported an AsyncIterable as input, so that we could maintain good generics that we lose if using Readable.from.