Update readme

Author: polldo

README.md

@@ -123,4 +123,8 @@ The default OTA upload should complete in 10 minutes. Use `--deferred` flag to e
 
 Print a list of available dashboards and their widgets by using this command:
 
-`$ arduino-cloud-cli dashboard list --show-widgets`
+`$ arduino-cloud-cli dashboard list --show-widgets`
+
+Delete a dashboard with the following command:
+
+`$ arduino-cloud-cli dashboard delete --id <dashboardID>`