Add Apache license header

Author: GauthamBanasandra

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/WEB-INF/wro.properties

@@ -1,2 +1,16 @@
+#
+# Licensed 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. See accompanying LICENSE file.
+#
+
 groups.vendor=vendor.js
 groups.yarn-ui=yarn-ui.js