CLOUDSTACK-4334. DOC. Fix missing license header.

docs/en-US/storage-plugins.xml

@@ -3,6 +3,25 @@
 <!ENTITY % BOOK_ENTITIES SYSTEM "cloudstack.ent">
 %BOOK_ENTITIES;
 ]>
+
+<!-- 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.
+-->
+
 <chapter id="storage-plugins">
     <title>Writing a Storage Plugin</title>
     <para>This section gives an outline of how to implement a plugin