chore(storage): add warning in doc.go (googleapis#867)

googleapis/storage/v2/doc.go

@@ -0,0 +1,18 @@
+// Copyright 2022 Google LLC
+//
+// 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.
+
+// Warning: This package is not ready for production use and does not receive
+// regular updates. Usage of this code is not supported. This code may be
+// removed at will. Do not depend on it.
+package storage