WIP: contributing guidelines

CONTRIBUTING.md

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+# Contributing to AI Router
+
+## Found a bug?
+
+If you think you found a bug, please check [Issues](https://github.com/toverainc/ai-router/issues) first, to see if it's not already reported. If the bug is already reported, avoid adding comments like that add no additional information like "I am experiencing the same bug". Instead, add a thumbs up reaction to the main comment.
+
+When opening a new issue, please use a clear title and description. Include a minimal config needed to reproduce the bug, and extra details about the backend if needed (e.g. vLLM v0.4.2 with mistralai/Mistral-7B-Instruct-v0.1).
+
+## Code changes
+
+Open a Github PR with your changes. If your changes fix an existing issue, [link the PR to the issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue).
+
+### Code formatting
+
+Make sure your code is properly formatted by running `cargo fmt`. If your code is not properly formatted, CI will fail.
+
+### Tests
+
+Ideally you should add a test for the problem you are fixing, so that the problem will not reappear in the future.
+Also make sure your change does not break any of the existing tests by running `cargo test`. CI will also run the tests, and fail if any of the tests fail.
+
+### Clippy
+
+While some people consider Clippy to be a nuisance and it slows down development, we feel it helps to write better and more consistent Rust code. Therefore, we enable Clippy in CI. Please run `cargo clippy --all-features --all-targets --no-deps -- -Dclippy::pedantic` and make sure it succeeds.