docs: add CONTRIBUTING.md

Resolves #3743.

Pull-Request: #4134.

CONTRIBUTING.md

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+# Contributing Guidelines
+
+[![](https://img.shields.io/badge/made%20by-Protocol%20Labs-blue.svg?style=flat-square)](http://ipn.io)
+[![](https://img.shields.io/badge/project-libp2p-blue.svg?style=flat-square)](https://libp2p.io/)
+
+Welcome to the rust-libp2p contribution guide! We appreciate your interest in improving our library.
+
+## Looking for ways to contribute?
+
+There are several ways you can contribute to rust-libp2p:
+- Start contributing immediately via the opened [help wanted](https://github.com/libp2p/rust-libp2p/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22) or [difficulty:easy](https://github.com/libp2p/rust-libp2p/issues?q=is%3Aissue+is%3Aopen+label%3Adifficulty%3Aeasy) issues on GitHub.
+  These issues are suitable for newcomers and provide an excellent starting point.
+- Reporting issues, bugs, mistakes, or inconsistencies.
+  As many open source projects, we are short-staffed, we thus kindly ask you to be open to contribute a fix for discovered issues.
+
+### We squash-merge pull Requests
+
+We always squash merge submitted pull requests.
+This means that we discourage force pushes, in order to make the diff between pushes easier for us to review.
+Squash merging allows us to maintain a clean and organized commit history.
+
+The PR title, which will become the commit message after the squashing process, should follow [conventional commit spec](https://www.conventionalcommits.org/en/v1.0.0/).
+
+### Write changelog entries for user-facing changes
+
+When making user-facing changes, it is important to include corresponding entries in the changelog, providing a comprehensive summary for the users.
+For detailed instructions on how to write changelog entries, please refer to the documentation in [`docs/release.md`](https://github.com/libp2p/rust-libp2p/blob/master/docs/release.md).
+
+
+### Merging of PRs is automated
+
+To streamline our workflow, we utilize Mergify and the "send-it" label.
+Mergify automates merging actions and helps us manage pull requests more efficiently.
+The "send-it" label indicates that a pull request is ready to be merged.
+Please refrain from making further commits after the "send-it" label has been applied otherwise your PR will be dequeued from merging automatically.
+
+### Treat CI as a self-service platform
+
+We have a lot of automated CI checks for common errors.
+Please treat our CI as a self-service platform and try to fix any issues before requesting a review.