Improves documentation: Explain how to create a custom stanza extension

_examples/custom_stanza/README.md

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+# Custom Stanza example
+
+This module show how to implement a custom extension for your own client, without having to modify or fork Fluux XMPP.
+
+It help integrating your custom extension in the standard stream parsing, marshalling and unmarshalling workflow.

_examples/custom_stanza/custom_stanza.go

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+package main
+
+import (
+	"encoding/xml"
+	"fmt"
+	"log"
+
+	"gosrc.io/xmpp/stanza"
+)
+
+func main() {
+	iq := stanza.NewIQ(stanza.Attrs{Type: stanza.IQTypeGet, To: "service.localhost", Id: "custom-pl-1"})
+	payload := CustomPayload{XMLName: xml.Name{Space: "my:custom:payload", Local: "query"}, Node: "test"}
+	iq.Payload = payload
+
+	data, err := xml.Marshal(iq)
+	if err != nil {
+		log.Fatalf("Cannot marshal iq with custom payload: %s", err)
+	}
+
+	var parsedIQ stanza.IQ
+	if err = xml.Unmarshal(data, &parsedIQ); err != nil {
+		log.Fatalf("Cannot unmarshal(%s): %s", data, err)
+	}
+
+	parsedPayload, ok := parsedIQ.Payload.(*CustomPayload)
+	if !ok {
+		log.Fatalf("Incorrect payload type: %#v", parsedIQ.Payload)
+	}
+
+	fmt.Printf("Parsed Payload: %#v", parsedPayload)
+
+	if parsedPayload.Node != "test" {
+		log.Fatalf("Incorrect node value: %s", parsedPayload.Node)
+	}
+}
+
+type CustomPayload struct {
+	XMLName xml.Name `xml:"my:custom:payload query"`
+	Node    string   `xml:"node,attr,omitempty"`
+}
+
+func (c CustomPayload) Namespace() string {
+	return c.XMLName.Space
+}
+
+func init() {
+	stanza.TypeRegistry.MapExtension(stanza.PKTIQ, xml.Name{"my:custom:payload", "query"}, CustomPayload{})
+}