Add "comparison to may" to readme

[Leandros]

As a curious reader, it would be interesting to have a comparison to the other popular coroutine library written in Rust.

[Amanieu]

May is a high-level framework that uses https://github.com/Xudong-Huang/generator-rs internally for the coroutine implementation. Corosensei only provides the conroutine implementation, and not the full framework that may provides.