Add a chapter to the book about using gluon in an Async environment

[Ekleog]

Speaking of what came up in #537: binding FutureResult-returning functions into gluon and using .run_expr_async / .call_async to get a future that'll execute the code.

Maybe also speak of std.thread so that people know how to execute two (blocking from the gluon perspective, async from the rust perspective) operations at the same time.

I'll try to write such a chapter and PR it in as soon as I've got some time, if you haven't done it before I could. :)

[naturallymitchell]

Would this also have any possible relation to Actix, perhaps in terms of https://github.com/actix/actix/issues/79#issuecomment-389661547 ?

[Ekleog]

I don't really see why solving this issue (ie. adding a chapter in the gluon book) would have a possible relation to actix (didn't know of it before, knowing it only from the README), but as basically this chapter would say “gluon works nice with async, here's how, feel free to use it”, I guess gluon could indeed be used as an actix actor if someone wrote code for it? :)