Include CI badge in README

Author: khvzak

README.md

@@ -1,4 +1,8 @@
 # lua-src
+[![Build Status]][github-actions]
+
+[Build Status]: https://github.com/khvzak/lua-src-rs/workflows/CI/badge.svg
+[github-actions]: https://github.com/khvzak/lua-src-rs/actions
 
 This crate contains the sources of Lua 5.1/5.2/5.3/5.4(rc3) and logic to build them.
 Intended to be consumed by the [mlua](https://crates.io/crates/mlua) crate.