Create CONTRIBUTING.md

CONTRIBUTING.md

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+## Contributing
+
+[fork]: https://github.com/github/sonic-pi-challenge/fork
+[pr]: https://github.com/github/sonic-pi-challenge/compare
+[style]: https://github.com/styleguide/ruby
+[code-of-conduct]: CODE_OF_CONDUCT.md
+
+Please note that this project is released with a [Contributor Code of Conduct][code-of-conduct]. By participating in this project you agree to abide by its terms.
+
+## Submitting a pull request
+
+0. [Fork][fork] and clone the repository
+0. Configure and install the dependencies: `script/bootstrap`