Add upgrade guide

UPGRADE.md

@@ -1281,4 +1281,10 @@ export default ({
     sagaMiddleware.run(saga);
     return store;
 };
-```
+```
+
+## No More Tree Packages in Core
+
+The `ra-tree` and `ra-tree-ui-material-ui` packages were removed in v3. The v2 version performed poorly, and we don't want to delay v3 to reimplement the Tree packages.
+
+If you were using these packages just for displaying a tree, you'll have to reimplement a basic tree widget, taking the Tags list from the Simple example as an inspiration. If you were using these packages for creating and updating a tree, we recommend that you wait until the core team or another community member publishes a Tree packahe compatible with v3.