add info about version 2 on readme

README.md

@@ -22,3 +22,11 @@ To auto-generate Ruby code from a proto file, use the `protoc` plugin and the `-
 ## Contributing
 
 [On the CONTRIBUTING file](CONTRIBUTING.md).
+
+### Version 1 to 2
+
+The upgrade should be mostly compatible.
+The biggest change is that version 2 now allows Faraday 2.x as the http client.
+
+For more information on the changes, see [changeset](https://github.com/github/twirp-ruby/compare/v1.9.0...v2.0.0).
+