update plugins doc

Signed-off-by: Nora Blomaard <[email protected]>

Author: noraeb

docs/plug-ins.md

@@ -277,6 +277,14 @@ implemented in these plugins.
 > https://sprinteins.github.io/oscd-plugins/documentation/index.js
 > ```
 
+### Designer
+
+> <kbd>Editor</kbd> | by [OpenEnergyTools](https://github.com/OpenEnergyTools) | [Repository](https://github.com/OpenEnergyTools/oscd-designer)
+> 
+> ```
+> https://openenergytools.github.io/oscd-designer/oscd-designer.js
+> ```
+
 ## Vendor Specific Plug-ins
 
 These plug-ins are specific to a vendor and can include vendor specific