chore: add CONTRIBUTING.md

Author: rexagod

CONTRIBUTING.md

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+# Contributing
+
+Thank you for taking an interest in the project! We welcome all manner of contributions that are within the bounds of the project's [code of conduct](CODE_OF_CONDUCT.md).
+
+#### **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](SECURITY.md).
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/kubernetes-monitoring/kubernetes-mixin/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/kubernetes-monitoring/kubernetes-mixin/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, and a **`jsonnet` snippet**, if applicable, as well as an optional **visual sample** demonstrating the expected behavior that is not occurring.
+
+* Whenever possible, use the relevant bug report templates to create the issue.
+
+#### **Did you write a patch that fixes a bug?**
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description describes the problem **and** solution. Include the relevant issue number if applicable.
+
+* Before submitting, please make sure the pull request template is filled out correctly.
+
+#### **Do you intend to add a new feature or change an existing one?**
+
+* Suggest your change in [#monitoring-mixins](https://kubernetes.slack.com/archives/CAX9GU941) and start writing code. While doing so, please reflect on:
+  * Is your feature request related to a problem? Please describe the necessity for the change.
+  * Describe the solution you're proposing. Please provide any relevant context.
+  * Add any other context (for example, any workarounds, code snippets, visual aids, etc.), if applicable.
+
+* Do not open an issue on GitHub until you have collected positive feedback about the change. GitHub issues are primarily intended for bug reports and fixes.
+
+#### **Do you have questions about the source code?**
+
+* Ask any question about how to use the `kubernetes-mixin` project in the [#monitoring-mixins](https://kubernetes.slack.com/archives/CAX9GU941).
+
+---
+
+`kubernetes-mixin` is a volunteer effort. We encourage you to pitch in and join [the team](https://github.com/kubernetes-monitoring/kubernetes-mixin/graphs/contributors)!