Create CONTRIBUTING.md

Signed-off-by: Marek Kaput <marek.kaput@swmansion.com>

Author: mkaput

CONTRIBUTING.md

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+# Contribution Guideline
+
+Scarb is actively developed and open for contributions!
+Want to get started?
+Grab any unassigned issue labeled with [`good-first-issue`](https://github.com/orgs/software-mansion/projects/4/views/9)!
+Need some guidance?
+Reach out to other developers on [Telegram](https://t.me/+d8ULaPxeRqlhMDNk) or open a [discussion](https://github.com/software-mansion/scarb/discussions) on GitHub!
+
+## Environment setup
+
+Latest stable Rust is the only thing you really need.
+It is recommended to use [rustup](https://rustup.rs/) for getting it.
+
+If you wish to work on Scarb's website, you will need [Node.js](https://nodejs.org/).
+We recommend to install it using [asdf](https://asdf-vm.com/) (via [nodejs](https://github.com/asdf-vm/asdf-nodejs) plugin).
+
+## Contributing
+
+Before you open a pull request, it is always a good idea to search the issues and verify if the feature you would like
+to add hasn't been already discussed.
+We also appreciate creating a feature request before making a contribution, so it can be discussed before you get to
+work.
+
+### Writing Tests
+
+Please make sure the feature you are implementing is thoroughly tested with automatic tests.
+You can check test already in the repository, to see how to approach that.
+
+### Breaking Changes
+
+If the change you are introducing is changing or breaking the behavior of any already existing features, make sure to
+include that information in the pull request description.
+
+### Running Tests and Checks
+
+Before creating a contribution, make sure your code passes the following checks:
+
+```shell
+cargo fmt
+cargo clippy
+cargo test
+```
+
+Otherwise, it won't be possible to merge your contribution.
+
+### Git
+
+Try to make small PRs, that could be squashed into a single commit.
+For larger for, try to make your commits small, self-contained and well described.
+Each commit should pass lints and tests.
+We are using rebase for merging pull requests, and thus we do not allow merge commits.
+
+While your PR is being reviewed on, you can push merge commits and use [`git commit --fixup`](https://git-scm.com/docs/git-commit/2.32.0#Documentation/git-commit.txt---fixupamendrewordltcommitgt) to push further changes to your commits.
+Then, when your PR will be accepted, you can autosquash your fixups with [`git rebase --autosquash`](https://git-scm.com/docs/git-rebase#Documentation/git-rebase.txt---autosquash).
+
+---
+
+Thanks! ❤️ ❤️ ❤️
+
+Scarb Team