chore: update readme

SSShooter · SSShooter · commit 8aaf9800f983 · 2023-08-27T18:35:54.000+08:00
diff --git a/readme.md b/readme.md
@@ -34,6 +34,7 @@ Mind elixir is a free open source mind map core.
 - Pluginable
 - Build-in drag and drop / node edit plugin
 - Styling your node with CSS
+- Undo / Redo
 
 <details>
 <summary>Table of Contents</summary>
