docs: add link to the v2

Author: Lodin

README.md

@@ -9,6 +9,8 @@
 
 This package provides a lightweight and flexible solution for rendering large tree structures. It is built on top of the [react-window](https://github.com/bvaughn/react-window) library.
 
+**NOTE**: This is documentation for the `react-vtree` v3. For v2 see [this branch](https://github.com/Lodin/react-vtree/tree/version/2).
+
 **Attention!** This library is entirely rewritten to work with the `react-window`. If you are looking for the tree view solution for the [react-virtualized](https://github.com/bvaughn/react-virtualized), take a look at [react-virtualized-tree](https://github.com/diogofcunha/react-virtualized-tree).
 
 ## Installation