link regexpp from README

Author: qkniep

README.md

@@ -5,7 +5,7 @@
 ![LoC](https://tokei.rs/b1/github/qkniep/regexpp-rs?category=code)
 ![License](https://img.shields.io/github/license/qkniep/regexpp-rs)
 
-Rust port of regexpp (ECMAScript regular expression parser).
+Rust port of [regexpp](https://github.com/mysticatea/regexpp) (ECMAScript regular expression parser).
 This crate is being developed mainly as an efficient validator of ECMAScript regular expressions for
 the [`deno_lint` project](https://github.com/denoland/deno_lint).