Add documentation about annotation-filter (#448)

There were no documentation describing how to use the annotation-filter
parameter

docs/faq.md

@@ -200,3 +200,15 @@ spec:
         service
         ingress
 ```
+
+
+### Running an internal and external dns service
+
+Sometimes you need to run an internal and an external dns service.
+The internal one should provision hostnames used on the internal network (perhaps inside a VPC), and the external
+one to expose DNS to the internet.
+
+To do this with external-dns you can use the `--annotation-filter` to specifically tie an instance of external-dns to 
+an instance of a ingress controller. Let's assume you have two ingress controllers `nginx-internal` and `nginx-external`
+then you can start two external-dns providers one with `--annotation-filter=kubernetes.io/ingress.class=nginx-internal` 
+and one with `--annotation-filter=kubernetes.io/ingress.class=nginx-external`