Add disclaimer

packages/react-call-return/README.md

@@ -6,6 +6,21 @@ This is an experimental package for multi-pass rendering in React.
 
 **Use it at your own risk.**
 
+# No, Really, It Is Unstable
+
+This is **an experiment**.
+
+We will replace this with a different API in the future.  
+It can break between patch versions of React.
+
+Don't rely on this for anything except experiments.  
+Even in experiments, make sure to lock the versions so that an update doesn't break your app.
+
+Don't publish third party components relying on this unless you clearly mark them as experimental too.  
+They will break.
+
+Have fun! Let us know if you find interesting use cases for it.
+
 # API
 
 See the test case in `src/__tests__/ReactCallReturn.js` for an example.