docs(readme): add badge to readme

Author: tottoto

README.md

@@ -1,5 +1,8 @@
 # rust http headers
 
+[![crates.io](https://img.shields.io/crates/v/headers.svg)](https://crates.io/crates/headers)
+[![Released API docs](https://docs.rs/headers/badge.svg)](https://docs.rs/headers)
+[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)
 [![Build Status](https://github.com/hyperium/headers/workflows/CI/badge.svg)](https://github.com/hyperium/headers/actions?query=workflow%3ACI)
 
 Typed HTTP headers.