Add note about Relay Modern support [ci skip]

README.md

@@ -1,6 +1,8 @@
 # react-router-relay [![Travis][build-badge]][build] [![npm][npm-badge]][npm]
 [Relay](http://facebook.github.io/relay/) integration for [React Router](https://github.com/reactjs/react-router).
 
+**This library only supports Relay Classic. For Relay Modern support, see [Found Relay](https://github.com/4Catalyzer/found-relay).**
+
 **This library does not support React Router v4, because React Router v4 does not provide the necessary integration points for efficient data fetching with Relay. Additionally, rendering performance will be better with React Router v2 than with React Router v3, as the link subscription logic in React Router v3 triggers unnecessary rerenders with Relay.**
 
 [![Discord][discord-badge]][discord]