Update README

Author: Steve Ayers

README.md

@@ -52,6 +52,12 @@ To install the package, use `pip`:
 pip install protovalidate
 ```
 
+Protovalidate also has an optional dependency on [google-re2](https://pypi.org/project/google-re2/). If you require re2 syntax for your regular expressions, install the extra dep as follows:
+
+```shell
+pip install protovalidate[re2]
+```
+
 ## Documentation
 
 Comprehensive documentation for Protovalidate is available in [Buf's documentation library][protovalidate].