Add CONTRIBUTING.md guide (#215)

CONTRIBUTING.md

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+# Contributing to Heroku Cloud Native Buildpacks
+
+This page lists the operational governance model of this project, as well as
+the recommendations and requirements for how to best contribute to Heroku
+Cloud Native Buildpacks. We strive to obey these as best as possible. As
+always, thanks for contributing.
+
+## Governance Model: Salesforce Sponsored
+
+The intent and goal of open sourcing this project is to increase the contributor
+and user base. However, only Salesforce employees will be given `admin` rights
+and will be the final arbitrars of what contributions are accepted or not.
+
+## Getting started
+
+Please feel free to join the
+[Heroku Cloud Native Buildpacks discussions][discussions].
+You may also wish to take a look at
+[Heroku's product roadmap][roadmap] to see where are headed.
+
+## Ideas and Feedback
+
+Please use
+[Heroku Cloud Native Buildpacks discussions][discussions]
+to provide feedback, request enhancements, or discuss ideas.
+
+## Issues, Feature Requests, and Bug Reports
+
+Issues, feature requests, and bug reports are tracked via [GitHub issues on
+this repository][issues]. If you find
+an issue and/or bug, please search the issues, and if it isn't already tracked,
+create a new issue.
+
+## Fixes, Improvements, and Patches
+
+Fixes, improvements, and patches all happen via [GitHub Pull Requests on this
+repository][pulls]. If you'd like to
+improve the tests, you want to make the documentation clearer, you have an
+alternative implementation of something that may have advantages over the way
+its currently done, or you have any other change, we would be happy to hear
+about it. For trivial changes, send a pull request. For non-trivial changes,
+consider [opening an issue](#issues-feature-requests-and-bug-reports) to
+discuss it first instead.
+
+## Development
+
+### Dependencies
+
+This buildpack relies on [heroku/libcnb.rs][libcnb] to compile buildpacks. All
+[libcnb.rs dependencies][libcnb-deps] will need to be setup prior to building
+or testing this buildpack.
+
+### Building
+
+1. Run `cargo check` to download dependencies and ensure there are no
+   compilation issues.
+1. Build the buildpack with `cargo libcnb package`.
+1. Use the buildpack to build an app: `pack build sample-app --buildpack packaged/x86_64-unknown-linux-musl/debug/heroku_procfile --path /path/to/sample-app`
+
+### Testing
+
+- `cargo test` performs Rust unit tests.
+- `cargo test -- --ignored` performs all integration tests.
+
+## Code of Conduct
+Please follow our [Code of Conduct](CODE_OF_CONDUCT.md).
+
+## License
+By contributing your code, you agree to license your contribution under the
+terms of our project [LICENSE](LICENSE) and to sign the
+[Salesforce CLA](https://cla.salesforce.com/sign-cla).
+
+
+[discussions]: https://github.com/heroku/buildpacks/discussions
+[issues]: https://github.com/heroku/buildpacks-procfile/issues
+[libcnb]: https://github.com/heroku/libcnb.rs
+[libcnb-deps]: https://github.com/heroku/libcnb.rs#development-environment-setup
+[pulls]: https://github.com/heroku/buildpacks-procfile/pulls
+[roadmap]: https://github.com/heroku/roadmap