[Docs] Add CONTRIBUTING file with contributor guidelines

CONTRIBUTING.md

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+## How to contribute to RMG-Py
+
+Thank you for contributing to RMG-Py! Please take a moment to review our guidelines:
+
+#### **Did you find a bug?**
+
+* ** Example...**
+
+* Please include in the ISSUES page.
+
+* For more detailed information on submitting a bug report and creating an issue, visit our [reporting guidelines](https://example.com).
+
+#### **Did you write code that fixes a bug?**
+
+* Open a new GitHub PR to merge into the main branch.
+
+* Make sure the PR clearly describes the problem + solution. If applicable, include the relevant issue.
+
+* Your PR must pass unit tests, regression tests, and code coverage, and receive approval from a reviewer before it can be merged in.
+
+#### **Do you want to see a new feature added?**
+
+* Suggest your change in the Issues tab. 
+
+#### **Do you have questions?**
+
+* Email us at [email protected].
+
+#### **Do you want to contribute to the documentation?**
+
+* Example here...
+
+### Best practices for PRs:
+
+* Rebase to the main branch before working, to avoid merge conflicts.
+
+* Keep PRs small and aim to merge quickly.
+
+* Submit a PR only when the code is polished and ready for review. Consider opening a draft PR for work in progress that requires collaborator input.
+
+* For complex, multi-step PRs, consider opening "megathread" PRs pointing to the main branch. Then, create other PRs that merge into the "megathread" PR branch. This helps divide the review process into smaller chunks.
+
+
+Thank you!
+RMG Developers