docs(contributing): add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering contributing to [git-cliff](https://github.com/orhun/git-cliff/)!
+
+When contributing, please first discuss the change you wish to make via [issue](https://github.com/orhun/git-cliff/issues),
+[email](mailto:orhunparmaksiz@gmail.com), or any other method with the owners of this repository before making a change.
+
+Note that we have a [Code of Conduct](./CODE_OF_CONDUCT.md), please follow it in all your interactions with the project.
+
+## Setup
+
+1. Fork this repository and create your branch from `main`.
+
+2. Clone your forked repository.
+
+```sh
+git clone https://github.com/{username}/git-cliff && cd git-cliff
+```
+
+3. Make sure that you have [Rust](https://www.rust-lang.org/) `1.53.0-nightly` or later installed and build the project.
+
+```sh
+cargo build
+```
+
+4. Start committing your changes. Follow the [conventional commit specification](https://www.conventionalcommits.org/) while doing so.
+
+5. Add your tests (if you haven't already) or update the existing tests according to the changes. And check if the tests are passed.
+
+```sh
+cargo test
+```
+
+7. Make sure [rustfmt](https://github.com/rust-lang/rustfmt) and [clippy](https://github.com/rust-lang/rust-clippy) don't complain about your changes.
+
+```sh
+cargo fmt --all -- --check --verbose
+cargo clippy --verbose -- -D warnings
+```
+
+## Create a Pull Request
+
+1. Ensure that you updated the documentation and filled the [Pull Request template](./.github/PULL_REQUEST_TEMPLATE.md) according to the changes you made.
+
+2. Wait for approval from the project owner/maintainer. Discuss the possible changes and update your Pull Request if necessary.
+
+3. You may merge the Pull Request once you have the sign-off of the project owner/maintainer, or if you do not have permission to do that, you may request the project owner/maintainer to merge it in case they haven't done it after a while.
+
+# License
+
+By contributing, you agree that your contributions will be licensed under [GNU General Public License v3.0](./LICENSE).