add license header

Signed-off-by: Patrick Pan <[email protected]>

src/OrasProject.Oras/Exceptions/ReferrersSupportLevelAlreadySetException.cs

@@ -1,4 +1,17 @@
-using System;
+// Copyright The ORAS Authors.
+// 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.
+
+using System;
 
 namespace OrasProject.Oras.Exceptions;