add license to .mailmap

ATR was complaining

.mailmap

@@ -1,3 +1,20 @@
+# 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.
+
 # .mailmap unifies differences in commit names and emails, and shortens committers' names.
 # .mailmap format is a bit weird, e.g. https://github.com/sympy/sympy/wiki/Using-.mailmap explains it.
 # .mailmap official doc is on https://git-scm.com/docs/git-shortlog#_mapping_authors