Add zenoh to the list of async-std ecosystem

[zenoh](https://crates.io/crates/zenoh) is an innovative communication framework entirely written in async Rust. We focus on performance, safety and usability. We think that [zenoh](https://crates.io/crates/zenoh) fills an important gap in the async Rust ecosystem and as such would like to be added to the list.

I will submit a pull request with the suggested changed to the readme.

Locate the README section listing the async-std ecosystem projects and review its existing formatting and links. Add zenoh with its crates.io link, then verify that the entry matches the surrounding list.

Written by the indexing model from the issue text.