add contributing doc page.

CONTRIBUTING.md

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+# Contributing
+
+## Before you get started
+
+### Code of Conduct
+
+Please make sure to read and observe our Code of Conduct.
+
+### Install requirements
+- Python `3.11`
+  - poetry `1.8.4` & up
+- A LLM API key is required to use and test HolmesGPT
+  - OpenAI's `gpt4-o` is recommended.
+  - For details see [Getting an API Key](https://github.com/robusta-dev/holmesgpt?tab=readme-ov-file#getting-an-api-key).
+
+## Reporting bugs
+
+We encourage those interested to contribute code and also appreciate when issues are reported. 
+
+- Create a new issue and label is as `bug`
+- Clearly state how to reproduce the bug:
+  - Which LLM you've used
+  - Which steps are required to reproduce
+    - As LLMs answers may differ between runs - Does it always reproduce, or occasionally?
+
+
+## Contributing Code
+
+- Fork the repository and clone it locally.
+- Create a new branch and make your changes
+- Add or update tests to ensure your changes are covered.
+- Run `pytest` to verify all tests pass.
+- Keep pull requests small and focused. if you have multiple changes, open a PR for each.
+- Create a pull request back to the upstream repository.
+- Wait for a review and address any comments