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

Author: nudles

doc/en/releases/RELEASE_NOTES_2.0.0.md

@@ -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
+
+      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.
+-->
+
 # singa-incubating-2.0.0 Release Notes
 
 ---

rat-excludes

@@ -1 +1,3 @@
-rat-excludes
+rat-excludes
+# Apache Rat cannot recognize the license header in jupyter notebooks
+.*\.ipynb