README: Document how to set up the incoming email server.

Fixes: #237.

Author: alexmv

README.md

@@ -224,6 +224,13 @@ lists of IP addresses or CIDR ranges.
 
 [smokescreen]: https://zulip.readthedocs.io/en/latest/production/deployment.html#customizing-the-outgoing-http-proxy
 
+**Incoming email**: The Docker image exposes port 25, which is already
+configured with Zulip's incoming email server.  To use it, publish
+port 25 of the Docker container, set `SETTING_EMAIL_GATEWAY_PATTERN`,
+and add an MX record to your DNS configuration pointing to the Docker
+container's public hostname (or wherever you chose to publicly expose
+its port 25).
+
 ### Manual configuration
 
 The way the environment variables configuration process described in