Update readme with newest feature

Author: mzur

README.md

@@ -18,6 +18,7 @@ This is a clone of the [Workspace Grid](https://github.com/zakkak/workspace-grid
 - Workspace labels in the workspace switcher popup (optional).
 - Workspace overview on <kbd>Super</kbd>+<kbd>W</kbd>.
 - Workspace switcher popup on all monitors (optional).
+- Workspace grid in the activity overview (optional).
 
 ## Installation