Added note on client-side extension point

Author: AndyButland

16/umbraco-cms/customizing/back-office-signs.md

@@ -33,4 +33,7 @@ Using this binding lets the server determine which signs are present in the resp
 
 ## Displaying a Sign
 
-TBC
+{% hint style="info" %}
+The client extension for backoffice signs is due in Umbraco 16.4 / 17.0.
+{% endhint %}
+