Update README.md

Author: mike-solomon

README.md

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 This project implements a [Rewrite module](https://github.com/openrewrite/rewrite) that performs common Java testing migration tasks, like migrating from JUnit 4 to JUnit 5.
 
 Browse [a selection of recipes available through this module in the recipe catalog](https://docs.openrewrite.org/recipes/java/testing).
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+## Contributing
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+We appreciate all types of contributions. See the [contributing guide](https://github.com/openrewrite/.github/blob/main/CONTRIBUTING.md) for detailed instructions on how to get started.