Update recommended coredns_omada version for 6.20.x

Author: dougbw

README.md

@@ -26,5 +26,6 @@ Docker container images are now being published to the [GitHub Container Registr
 
 | Omada Controller version  | Reccomended coredns_omada version |
 | --------                  | -------               |
+| `6.20.x`                  | `v1.9.1` or newer     |
 | `5.12.x`                  | `v1.5.0` or newer     |
 | `5.9.x`                   | `v1.4.2`              |