feat(docs): add CONTRIBUTING.md file

Signed-off-by: Tarek <[email protected]>

CONTRIBUTING.md

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+You made it here! This is a great step in helping to contribute to ARK 🎈
+
+## How to contribute
+
+To get started, you can start off here [issues](https://github.com/ARK-Builders/ark-rust/issues) with those tagged [`good first issue`](https://github.com/ARK-Builders/ark-rust/issues?q=is:issue+is:open+label:%22good+first+issue%22).
+
+You can find fresh builds as artifacts of [GitHub Actions workflows](https://github.com/ARK-Builders/ark-rust/actions):
+
+- The "Verify build" workflow runs tests on supported platforms
+- Benchmarks are run on every PR. It uses [`criterion`](https://github.com/bheisler/criterion.rs) to measure performance of the code compared to current main branch
+
+## Forking the project
+
+Before we can add you as a contributor to our project, we suggest to do initial work from your own fork of the project.
+
+To create a fork, please press `fork` button on the project page:
+
+Then you can modify everything without fear of breaking official version.
+
+## Submitting a Pull Request
+
+After you've implemented a feature or fixed a bug, it is time to open Pull Request.
+
+Please enable GitHub Actions in your fork, so our QA will be able to download build of your version without manually compiling from source code.
+
+### Automated code style checks
+
+The projects uses `rustfmt` and `clippy` to enforce code style and best practices. You can run them locally with:
+
+```bash
+cargo fmt --all
+cargo clippy --workspace --bins -- -D warnings
+```
+
+### Code review
+
+We care a lot about our software quality, that's why we are conducting strict code reviews before merging:
+
+- we will ask questions if we are not sure about particular technical decision
+- when possible, we will suggest alternative solution
+- GitHub Actions workflow must result in success (be green)
+- comments must be resolved before merge
+- code style should be green as well
+
+Right now, the team isn't that big, so please be patient 🙂
+
+### Merge conflicts
+
+If Pull Request is long time in reviewing phase, `main` branch might go forward too far.
+Please, fix all merge conflicts in this case 🛠
+
+## Additional read
+
+https://docs.github.com/en/get-started/quickstart/github-flow