Update readme

Author: amoeba

README.md

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 Lint your Ruby code in parallel to your builds with [StandardRB](https://github.com/testdouble/standard).
 Based almost entirely off the [Rubucop Action](https://github.com/gimenete/rubocop-action) by [Alberto Gimeno](https://github.com/gimenete).
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+![](docs/images/screenshot.png)
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+## Contributing
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+Please file an [Issue](https://github.com/amoeba/standardrb-action) for bug reports, feature requests, or other comments.