Add README for component.

cmd/xmpp_component/README.md

@@ -0,0 +1,20 @@
+# xmpp_component
+
+This component will connect to ejabberd and act as a subdomain "service" of your primary XMPP domain
+(in that case localhost).
+
+To be able to connect this component, you need to add a listener to your XMPP server.
+
+Here is an example ejabberd configuration for that component listener:
+
+```yaml
+listen:
+...
+  -
+    port: 8888
+    module: ejabberd_service
+    password: "mypass"
+```
+
+ejabberd will listen for a component (service) on port 8888 and allows it to connect using the
+secret "mypass".