Update .gitignore -- legalese header.

.gitignore

@@ -1,3 +1,20 @@
+# Flow
+# Copyright 2023 Akamai Technologies, Inc.
+#
+# 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
+#
+# https://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.
+
 # Ignore emacs auto-saves and recovery files; **/ to avoid picking it up as a comment.
 *~
 **/#*#