XEP-0388: Add preliminary schema

I'll bet I've cocked this up too.

xep-0388.xml

@@ -469,6 +469,93 @@
   <p>None.</p>
 </section1>
 
+  <section1 topic="XML Schema">
+    <code><![CDATA[
+<?xml version='1.0' encoding='utf-8'?>
+<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema'
+           targetNamespace="urn:xmpp:sasl:1"
+           xmlns="urn:xmpp:sasl:1"
+           elementFormDefault="qualified">
+
+    <xs:element name="mechanisms">
+        <xs:complexType>
+            <xs:sequence>
+                <xs:element name="mechanism" type="SaslMechName" minOccurs="1" maxOccurs="unbounded"/>
+                <xs:any/>
+            </xs:sequence>
+        </xs:complexType>
+    </xs:element>
+
+    <xs:element name="authenticate">
+        <xs:complexType>
+            <xs:attribute name="mechanism" type="SaslMechName"/>
+            <xs:sequence>
+                <xs:element name="initial-response"/>
+                <xs:any/>
+            </xs:sequence>
+        </xs:complexType>
+    </xs:element>
+
+    <xs:element name="next">
+        <xs:complexType>
+            <xs:attribute name="task" type="SaslMechName"/>
+            <xs:sequence>
+                <xs:element name="initial-response"/>
+                <xs:any/>
+            </xs:sequence>
+        </xs:complexType>
+    </xs:element>
+
+    <xs:element name="challenge" type="SaslData"/>
+
+    <xs:element name="response" type="SaslData"/>
+
+    <xs:element name="initial-response" type="SaslData"/>
+
+    <xs:element name="success">
+        <xs:complexType>
+            <xs:sequence>
+                <xs:element name="additional-data" type="SaslData"/>
+                <xs:element name="authorization-identifier" type="Jid"/>
+            </xs:sequence>
+        </xs:complexType>
+    </xs:element>
+
+    <xs:element name="continue">
+        <xs:complexType>
+            <xs:sequence>
+                <xs:element name="additional-data" type="SaslData"/>
+                <xs:element name="tasks">
+                    <xs:complexType>
+                        <xs:sequence>
+                            <xs:element name="tasK" type="SaslMechName"/>
+                        </xs:sequence>
+                    </xs:complexType>
+                </xs:element>
+            </xs:sequence>
+        </xs:complexType>
+    </xs:element>
+
+    <xs:simpleType name="Jid">
+        <xs:restriction base="xs:string">
+            <xs:maxLength value="3071"/>
+        </xs:restriction>
+    </xs:simpleType>
+
+    <xs:simpleType name="SaslMechName">
+        <xs:restriction base="xs:string">
+            <xs:minLength value="1"/>
+            <xs:maxLength value="20"/>
+        </xs:restriction>
+    </xs:simpleType>
+
+    <xs:simpleType name="SaslData">
+        <xs:restriction base="xs:base64Binary"/>
+    </xs:simpleType>
+</xs:schema>
+]]></code>
+  </section1>
+
 <section1 topic='Acknowledgements' anchor='ack'>
   <p>The author wishes to share any credit with many members of the community, including Lance Stout, Ralph Meijer, Phil Roberts and Florian Schmaus.</p>
 </section1>