Rationalize copyright headers and add the contributing files

The headers were modified with
`find . -type f -exec 'sed' '-Ei' 's|Copyright 201([45]) Google Inc|Copyright 201\1 The Bazel Authors|' '{}' ';'`
diff --git a/gerrit-github-sync/gerrit-github-sync.sh b/gerrit-github-sync/gerrit-github-sync.sh
index 205d4d3..af2dae4 100644
--- a/gerrit-github-sync/gerrit-github-sync.sh
+++ b/gerrit-github-sync/gerrit-github-sync.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 #
-# Copyright 2015 Google Inc. All rights reserved.
+# Copyright 2015 The Bazel Authors. 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.