Add CONTRIBUTING.md

Signed-off-by: Sergey Minaev <[email protected]>

CONTRIBUTING.md

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+# Contributing to SD-JWT
+
+We love your input! We want to make contributing to this project as easy and transparent as possible, whether it's:
+
+- Reporting a bug
+- Discussing the current state of the code
+- Submitting a fix
+- Proposing new features
+- Becoming a maintainer
+
+## We Develop with Github
+
+We use github to host code, to track issues and feature requests, as well as accept pull requests.
+
+## We Use [Github Flow](https://guides.github.com/introduction/flow/index.html), So All Code Changes Happen Through Pull Requests
+
+Pull requests are the best way to propose changes to the codebase (we use [Github Flow](https://guides.github.com/introduction/flow/index.html)). We actively welcome your pull requests:
+
+1. Fork the repo and create your branch from `main`.
+2. If you've added code that should be tested, add tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints.
+6. Issue that pull request!
+
+## Any contributions you make will be under the Apache 2.0 Software License
+
+In short, when you submit code changes, your submissions are understood to be under the same [Apache 2.0 License](http://www.apache.org/licenses/) that covers the project. Feel free to contact the maintainers if that's a concern.
+
+## Report bugs using Github's [issues](https://github.com/openwallet-foundation-labs/sd-jwt-rust/issues)
+
+We use GitHub issues to track public bugs. Report a bug by opening a new issue it's that easy!
+
+## Write bug reports with detail, background, and sample code
+
+**Great Bug Reports** tend to have:
+
+- A quick summary and/or background
+- Steps to reproduce
+    - Be specific!
+    - Give sample code if you can.
+- What you expected would happen
+- What actually happens
+- Notes (possibly including why you think this might be happening, or stuff you tried that didn't work)