docs: add contributing guidelines file

CONTRIBUTING.md

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+I welcome contributions of any kind with open arms. That said, please do stick to some basics:
+
+- For code contributions:
+  - Fix any `cargo clippy` warnings, using at least the default configuration.
+  - Make sure your code is formatted using `cargo fmt`.
+  - Keep any documentation up to date.
+  - I won't enforce it, but preferably stick to [conventional commit](https://www.conventionalcommits.org/en/v1.0.0/) messages.
+
+
+- For PRs:
+  - Please open an issue or discussion beforehand. 
+    I'll accept most contributions, but it's best to make sure you're not working on something that won't get accepted :)
+
+
+- For issues:
+  - Please provide as much information as you can - share your config, any logs, steps to reproduce...