docs(csharp): add Apache 2.0 license header to release process doc

Co-authored-by: Isaac

Author: eric-wang-1990

csharp/RELEASE_PROCESS.md

@@ -1,3 +1,19 @@
+<!--
+  Copyright (c) 2025 ADBC Drivers Contributors
+
+  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.
+-->
+
 # C# Driver Release Process
 
 ## Overview