add example which uses filt

examples/README

@@ -3,6 +3,7 @@ add_header.py             Simple script that just adds a header to every request
 change_upstream_proxy.py  Dynamically change the upstream proxy
 dns_spoofing.py           Use mitmproxy in a DNS spoofing scenario.
 dup_and_replay.py         Duplicates each request, changes it, and then replays the modified request.
+filt.py                   Use mitmproxy's filter expressions in your script.
 iframe_injector.py        Inject configurable iframe into pages.
 modify_form.py            Modify all form submissions to add a parameter.
 modify_querystring.py     Modify all query strings to add a parameters.