Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+## How to contribute
+
+#### **Did you find a bug?**
+
+* **Do not open up a GitHub issue if the bug is a security vulnerability**, and instead to refer to our [security policy](https://github.com/CnCNet/cncnet-yr-client-package/blob/develop/SECURITY.md).
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/CnCNet/cncnet-yr-client-package/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/CnCNet/cncnet-yr-client-package/issues/new). Be sure to use our templates to help you submit an issue, please include as much relevant information as possible.
+
+#### **Did you fix a bug?**
+
+* Open a new GitHub pull request linked to the issue (or create an issue first and then link it).
+
+* Ensure the PR description clearly describes the problem and solution and ensure it is linked to an issue.
+
+All CnCNet projects including this one are contributed to on a volunteer effort.
+
+Thanks!