Add "async chapter" once the ecosystem around it stabilizes somewhat

[budziq]

https://internals.rust-lang.org/t/crate-evaluation-for-2017-08-15-mio/5709/7

I think it is in scope for the cookbook to help users decide what level of the async stack they want to operate at. For example I can imagine a dedicated async chapter showing characteristic code for mio, tokio, futures, async hyper, and gotham so users can get a sense of what is most appropriate for their use case. I closed the cookbook tracking issue because I don’t think we are ready for that yet, but let’s follow up later as the relevant parts of the ecosystem approach stability.

[randomPoison]

I would request that the cookbook demonstrate how to asynchronously load a file from disk, assuming there's a way to do so. As far as I know, there's currently no good solution for loading files asynchronously, but once there is, I think it would be a good addition to the cookbook.

[AndyGauge]

There's an RFC to add await/async to the language rust-lang/rfcs#2394 Should we describe the existing async story or wait until the language implements it? The Cookbook isn't really about language features though.

[budziq]

While Cookbook is more about batteries included experience around the stdlib and crates rather than language features.

I'd suggest to wait for the dust to settle around everything async. For now tokio is in the middle of a compatibility breaking reform and as noted the language side is still in flux.