Add schema file

A schema file might help remembering the new changelog format.

Author: ppkarwasz

log4j-changelog/src/main/resources/log4j-changelog.xsd

@@ -0,0 +1,73 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<schema xmlns="http://www.w3.org/2001/XMLSchema"
+  targetNamespace="https://logging.apache.org/log4j/log4j-changelog"
+  xmlns:cl="https://logging.apache.org/log4j/log4j-changelog" elementFormDefault="qualified">
+
+  <element name="release">
+    <complexType>
+      <simpleContent>
+        <extension base="string">
+          <attribute name="version" use="required"/>
+          <attribute name="date" use="required">
+            <simpleType>
+              <union memberTypes="date cl:tbd"/>
+            </simpleType>
+          </attribute>
+        </extension>
+      </simpleContent>
+    </complexType>
+  </element>
+
+  <simpleType name="tbd">
+    <restriction base="string">
+      <enumeration value="TBD"/>
+    </restriction>
+  </simpleType>
+
+  <element name="entry">
+    <complexType>
+      <sequence>
+        <element name="issue" type="cl:issueType" minOccurs="0" maxOccurs="unbounded"/>
+        <element name="author" type="cl:authorType" maxOccurs="unbounded"/>
+        <element name="description" type="cl:descriptionType"/>
+      </sequence>
+      <attribute name="type" type="cl:changeType" use="required"/>
+    </complexType>
+  </element>
+
+  <complexType name="issueType">
+    <simpleContent>
+      <extension base="string">
+        <attribute name="id" type="ID" use="required"/>
+        <attribute name="link" type="anyURI" use="required"/>
+      </extension>
+    </simpleContent>
+  </complexType>
+
+  <simpleType name="changeType">
+    <restriction base="string">
+      <enumeration value="added"/>
+      <enumeration value="changed"/>
+      <enumeration value="deprecated"/>
+      <enumeration value="removed"/>
+      <enumeration value="fixed"/>
+    </restriction>
+  </simpleType>
+
+  <complexType name="authorType">
+    <simpleContent>
+      <extension base="string">
+        <attribute name="id" type="ID"/>
+        <attribute name="name"/>
+      </extension>
+    </simpleContent>
+  </complexType>
+
+  <complexType name="descriptionType">
+    <simpleContent>
+      <extension base="string">
+        <attribute name="type" use="required"/>
+      </extension>
+    </simpleContent>
+  </complexType>
+</schema>