added core extensions documentation

maven-core/src/main/resources/META-INF/maven/extension.xml

@@ -18,7 +18,7 @@ KIND, either express or implied.  See the License for the
 specific language governing permissions and limitations
 under the License.
 -->
-
+<!-- START SNIPPET: core-extension -->
 <extension>
   <exportedPackages>
     <!-- maven-* -->
@@ -161,7 +161,7 @@ under the License.
     <exportedArtifact>org.fusesource.jansi:jansi</exportedArtifact>
 
     <!--
-      | We must also filter out the old or NoClassDefFoundErrors will surface  
+      | We must also filter out the old Aether or NoClassDefFoundErrors will surface  
      -->
     <exportedArtifact>org.sonatype.aether:aether-api</exportedArtifact>
     <exportedArtifact>org.sonatype.aether:aether-spi</exportedArtifact>
@@ -174,4 +174,5 @@ under the License.
       | wagon from their plugin realm. 
      -->
   </exportedArtifacts>
-</extension>
+</extension>
+<!-- END SNIPPET: core-extension -->

maven-core/src/site/apt/core-extensions.apt.vm

@@ -0,0 +1,30 @@
+~~ 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.
+
+ ---
+ Maven Core Extensions Reference
+ ---
+ Hervé Boutemy
+ ---
+ 2016-06-25
+ ---
+
+Maven Core Extensions Reference
+
+  Maven core provides default extensions as defined in <<</META-INF/maven/extension.xml>>>:
+
+%{snippet|id=core-extension|file=${project.basedir}/src/main/resources/META-INF/maven/extension.xml}

maven-core/src/site/apt/index.apt

@@ -53,7 +53,7 @@ Maven Core
  ({{{./xref/org/apache/maven/classrealm/DefaultClassRealmManager.html}source}}), using
  {{{https://codehaus-plexus.github.io/plexus-classworlds/}Plexus Classworlds}},
 
- * {{{./extension.html}extension descriptor}},
+ * {{{./extension.html}extension descriptor}} and {{{./core-extensions.html}core extensions}},
 
  * <<<ExceptionHandler>>> component ({{{./apidocs/org/apache/maven/exception/ExceptionHandler.html}javadoc}}),
  with its <<<DefaultExceptionHandler>>> implementation

maven-core/src/site/site.xml

@@ -34,6 +34,7 @@ under the License.
       <item name="Lifecycles" href="lifecycles.html"/>
       <item name="Plugin Bindings to Default Lifecycle" href="default-bindings.html"/>
       <item name="Artifact Handlers" href="artifact-handlers.html"/>
+      <item name="Core Extensions" href="core-extensions.html"/>
     </menu>
 
     <menu ref="parent"/>