Add note on using auth host mode with selective auth

Author: thomseddon

README.md

@@ -367,6 +367,8 @@ spec:
       - name: traefik-forward-auth
 ```
 
+Note: If using auth host mode, you must apply the middleware to your auth host ingress.
+
 See the examples directory for more examples.
 
 #### Selective Container Authentication in Swarm
@@ -381,6 +383,8 @@ whoami:
     - "traefik.http.routers.whoami.middlewares=traefik-forward-auth"
 ```
 
+Note: If using auth host mode, you must apply the middleware to the traefik-forward-auth container.
+
 See the examples directory for more examples.
 
 #### Rules Based Authentication