chore: add CONTRIBUTING.md file

Author: BBarisKilic

CONTRIBUTING.md

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+# Contributing to Audio Player Service
+
+First off, thanks for taking the time to contribute!
+
+The following is a set of guidelines for contributing to the library and related packages. These are mostly guidelines, not rules. Use your best judgment, and feel free to propose changes to this document in a pull request.
+
+## Proposing a Change
+
+We recommend filing an issue if you intend to change the public API or make any non-trivial changes to the implementation. It lets us reach an agreement on your proposal before you put significant effort into it.
+
+If you’re only fixing a bug, it’s fine to submit a pull request right away, but we still recommend filing an issue detailing what you’re fixing. It is helpful if we don’t accept that specific fix but want to keep track of the issue.
+
+## Creating a Pull Request
+
+Before creating a pull request please:
+
+1. Fork the repository and create your branch from `main`.
+1. Install all dependencies (`flutter packages get` or `pub get`).
+1. Squash your commits and ensure you have a meaningful commit message.
+1. If you’ve fixed a bug or added code that should be tested, add tests!
+   Pull Requests without 100% test coverage will not be approved.
+1. Ensure the test suite passes.
+1. If you've changed the public API, make sure to update/add documentation.
+1. Format your code (`dart format .`).
+1. Analyze your code (`dart analyze --fatal-infos --fatal-warnings .`).
+1. Create the Pull Request.
+1. Verify that all status checks are passing.
+
+While the prerequisites above must be satisfied before having your pull request reviewed, the reviewer(s) may ask you to complete additional design work, tests, or other changes before your pull request can be ultimately accepted.
+
+## Getting in Touch
+
+If you want to just ask a question or get feedback on an idea, you can post it as a GitHub issue.
+
+## License
+
+By contributing to animated_svg, you agree that your contributions will be licensed under its [license](LICENSE).