Update README.md

Author: mikewiebe

README.md

@@ -52,6 +52,7 @@ vrf_delete_mode: false
 inventory_delete_mode: false
 vpc_delete_mode: false
 link_vpc_delete_mode: false
+policy_delete_mode: false
 ```
 
 **Note:** These variables are set to `false` by default to avoid accidental removal of configuration from NDFC that might impact the network.