fix: Added migration guide

Author: ellinge

docs/guides/migrating-from-1-to-2.md

@@ -0,0 +1,9 @@
+## React Dropdown Tree Select Migration Guide v1 to v2
+
+We've made some changes in the component between the versions which is covered in the following guide
+
+### Property changes
+
+| Description                                                              | Usage before                           | Usage after           |
+| ------------------------------------------------------------------------ | -------------------------------------- | --------------------- |
+| Added a new radio select mode<br>Selection is done in a shared mode-prop | `simpleSelect={true}` / `simpleSelect` | `mode='simpleSelect'` |