cleanup license headers and contributing docs

- switch to "project authors" style copyright.  Instead of an AUTHORS
  file (https://opensource.google/docs/releasing/authors/), I just list
  Google as a major copyright holder in the README.
- use SPDX style license headers in source files
- remove CLA requirement from contributing docs

imageproxy_test.go

@@ -1,16 +1,5 @@
-// Copyright 2013 Google LLC. All rights reserved.
-//
-// 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.
+// Copyright 2013 The imageproxy authors.
+// SPDX-License-Identifier: Apache-2.0
 
 package imageproxy