Add contributing guidelines to CONTRIBUTING.md

Added guidelines for contributing to the project, including steps for submitting a pull request and resources for contributors.

Author: patniko

CONTRIBUTING.md

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+## Contributing
+
+[fork]: https://github.com/github/copilot-plugins/fork
+[pr]: https://github.com/github/copilot-plugins/compare
+
+Hi there! We're thrilled that you'd like to contribute to this project. Your help is essential for keeping it great.
+
+Contributions to this project are [released](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license) to the public under the [project's open source license](LICENSE.txt).
+
+Please note that this project is released with a [Contributor Code of Conduct](CODE_OF_CONDUCT.md). By participating in this project you agree to abide by its terms.
+
+## Submitting a pull request
+
+1. [Fork][fork] and clone the repository
+1. Create a new branch: `git checkout -b my-branch-name`
+1. Make your change, add tests, and make sure the tests and linter still pass
+1. Push to your fork and [submit a pull request][pr]
+1. Pat yourself on the back and wait for your pull request to be reviewed and merged.
+
+Here are a few things you can do that will increase the likelihood of your pull request being accepted:
+
+- Keep your change as focused as possible. If there are multiple changes you would like to make that are not dependent upon each other, consider submitting them as separate pull requests.
+- Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+
+## Resources
+
+- [How to Contribute to Open Source](https://opensource.guide/how-to-contribute/)
+- [Using Pull Requests](https://help.github.com/articles/about-pull-requests/)
+- [GitHub Help](https://help.github.com)