add note about reverse proxies and firewalls for mailhog#117

docs/CONFIG.md

@@ -51,3 +51,12 @@ the following structure, and set `MH_OUTGOING_SMTP` or `-outgoing-smtp`.
 Only `name`, `host` and `port` are required.
 
 `mechanism` can be `PLAIN` or `CRAM-MD5`.
+
+### Firewalls and proxies
+
+If you have MailHog behind a firewall, you'll need ports `8025` and `8080` by default.
+
+You can override this using `-api-bind-addr`, `-api-bind-addr` and `-api-bind-addr` configuration options.
+
+If you're using MailHog behind a reverse proxy, e.g. nginx, make sure WebSocket connections
+are also supported and configured - see [this issue](https://github.com/mailhog/MailHog/issues/117) for information.