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

cmd/imageproxy-sign/main_test.go

@@ -1,3 +1,6 @@
+// Copyright 2013 The imageproxy authors.
+// SPDX-License-Identifier: Apache-2.0
+
 package main
 
 import (