Add Granian to the cited server options

[gi0baro]

Disclaimer: I'm the Granian maintainer

Granian is a Rust HTTP server for Python applications which supports ASGI and have some unique features.
It isn't as well known in the Blacksheep community as an option, thus citing it aside of Uvicorn and Hypercorn in the project README and docs might help with that.

@RobertoPrevato if you see some value in this, I can open the relevant PRs.

[RobertoPrevato]

Hi @gi0baro
Absolutely, yes I see value in that. The only reason I didn't mention Granian earlier is lack of time to practice with it.
I started being active again on GitHub just a few days ago, after a very long break.
If you have time to contribute with relevant PRs, I welcome your help.

PS. I recognize you immediately as the Granian's maintainer, no introduction is needed for me. :)