Add Google open source disclaimer

[danicat]

To comply with Google's OSS policy:

add the disclaimer "This is not an officially supported Google product" in an appropriate location such as the project's README file. add license headers to all source code files To add license headers use addlicense: https://github.com/google/addlicense

Installation:

go install github.com/google/addlicense@latest

Add licenses:

$GOPATH/bin/addlicense -c "Google LLC" -l apache .

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