Adds a log4j-docgen module

The module contains a commons XML model to document a JARs containing
Log4j plugins.

Part of apache/logging-log4j2#1955.

log4j-docgen/pom.xml

@@ -0,0 +1,63 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one or more
+  ~ contributor license agreements.  See the NOTICE file distributed with
+  ~ this work for additional information regarding copyright ownership.
+  ~ The ASF licenses this file to you under the Apache License, Version 2.0
+  ~ (the "License"); you may not use this file except in compliance with
+  ~ the License.  You may obtain a copy of the License at
+  ~
+  ~      http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>org.apache.logging.log4j</groupId>
+    <artifactId>log4j-tools-parent</artifactId>
+    <version>${revision}</version>
+    <relativePath>../log4j-tools-parent</relativePath>
+  </parent>
+
+  <artifactId>log4j-docgen</artifactId>
+
+  <properties>
+    <bnd.baseline.fail.on.missing>false</bnd.baseline.fail.on.missing>
+  </properties>
+
+  <build>
+    <plugins>
+
+      <plugin>
+        <groupId>org.codehaus.modello</groupId>
+        <artifactId>modello-maven-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>generate-model</id>
+            <goals>
+              <goal>java</goal>
+              <goal>stax-reader</goal>
+              <goal>stax-writer</goal>
+              <goal>xdoc</goal>
+              <goal>xsd</goal>
+            </goals>
+            <configuration>
+              <domAsXpp3>false</domAsXpp3>
+              <models>
+                <model>src/main/mdo/plugins-model.xml</model>
+              </models>
+              <version>0.1.0</version>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+
+    </plugins>
+  </build>
+
+</project>