Add chapter about async programming

[flip111]

[flip111]

Some resources:

Allocators that don't need std https://github.com/Nemo157/embrio-rs/ https://josh.robsonchase.com/embedded-executor-2/ https://github.com/japaric/no-std-async-experiments taken from: https://github.com/rust-embedded/wg/issues/23

Interesting read: http://blog.japaric.io/brave-new-io/#async-hal

https://www.bobbin.io/

https://docs.rs/embedded-hal/0.2.2/embedded_hal/

[SergioGasquez]

I think this would be very helpful to have, in my opinion, maybe it should not explain the details of async, for that we can just point to https://rust-lang.github.io/async-book/ or any other resource, but maybe we could explain frameworks like RTIC or Embassy