Add more client libraries

This repo only lists a few client-side HTTP libraries, while there are many that could be useful in various circumstances:
- [attohttpc](https://lib.rs/attohttpc) (lightweight, no async)
- [curl](https://lib.rs/curl) (bindings to libcurl)
- [isahc](https://lib.rs/isahc) (higher level http client built on curl)
- [minreq](https://lib.rs/minreq) (very lightweight, no async)
- [surf](https://lib.rs/surf) (multiple http backends, such as hyper, curl, `window.fetch` on wasm)
- [ureq](https://lib.rs/ureq) (lightweight, no async)