FEAT: Add contribution guide

Author: MatteuSan

.github/CONTRIBUTING.md

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+# Contributing to Pycord UI
+First, thank you for contributing. Contributions help keep the library up-to-date and make it better. Contributions are always welcome.
+
+The following is a set of guidelines for contributing to the guide. These are guidelines, not hard rules.
+
+## Submitting a Pull Request
+1. Make sure you are up-to-date with the latest changes. It is recommended you pull the latest changes from the master branch before working on your own.
+2. Make sure to test your changes before submitting a pull request.
+3. Make sure you follow proper grammar and spelling.
+4. Try to follow git conventions. If you are not familiar with git, you can find a good tutorial [here](https://www.conventionalcommits.org/en/v1.0.0/).
+
+### Licensing
+By submitting a pull request, you agree:
+
+1. That you are submitting your own work. You hold the copyright to your work.
+2. To transfer your copyright to the guide.
+3. To license your contribution under the MIT License.
+4. If you are found to be in fault with any of the above, we shall not be held responsible in any way after the pull request has been merged.