Add `spdlog-rs` link to README

[SpriteOvO]

Hi, I would like to add a link to my personal project spdlog-rs to the README. It is inspired by the C++ logging library spdlog and is designed to provide performance benefits.

Based on the benchmarks, it's probably the fastest logging crate yet. And it is also compatible with log crate.

I've been maintaining this project for 3 years and today I released its v0.4.0 version. I believe it is ready to be used by more users.