Add JXR plugin to get rid of WARNING Unable to locate Source XRef to link to (#952)

pom.xml

@@ -470,6 +470,11 @@
                 <artifactId>maven-pmd-plugin</artifactId>
                 <version>${pmd.version}</version>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jxr-plugin</artifactId>
+                <version>3.0.0</version>
+            </plugin>
         </plugins>
     </reporting>