Update the editor icons README to remove outdated information

editor/icons/README.md

-The icons here are optimized SVGs, because the editor renders the svgs at runtime, they need
-to be small in size, so they can be efficiently parsed.
+# Editor icons
 
-The original icons can be found at:
-https://github.com/godotengine/godot-design/tree/master/engine/icons
+This folder contains all the icons used by Godot editor (except for platform
+icons which are located in their respective platform folder).
 
-There you can find the optimizer script.
-
-If you add a new icon, please make a pull request to this repo:
-https://github.com/godotengine/godot-design/
-
-and store the optimized SVG version here.
+See [Editor icons](https://docs.godotengine.org/en/latest/development/editor/creating_icons.html)
+in the documentation for details on creating icons for the Godot editor.