SINGA-375 Add comments to rat-exclude file

the comment starts with #
ipynb files are excluded because apache rat cannot recognize the license
header in jupyter notebook files ending with ipynb
diff --git a/doc/en/releases/ b/doc/en/releases/
index 3a9159c..b50e288 100644
--- a/doc/en/releases/
+++ b/doc/en/releases/
@@ -1,3 +1,22 @@
+    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
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
 # singa-incubating-2.0.0 Release Notes
diff --git a/rat-excludes b/rat-excludes
index 0930056..dd28d62 100644
--- a/rat-excludes
+++ b/rat-excludes
@@ -1 +1,3 @@
\ No newline at end of file
+# Apache Rat cannot recognize the license header in jupyter notebooks
\ No newline at end of file