Update repository URLs in CONTRIBUTING.md

Author: maureeungaro

CONTRIBUTING.md

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+# Contributing to GEMC
+
+We welcome contributions of code, documentation, tests, examples, and bug reports. This guide explains how to propose changes via GitHub.
+
+> **Quickstart**
+> 1. Open an issue to discuss your idea or bug.
+> 2. Fork the repo and clone your fork.
+> 3. Create a feature branch.
+> 4. Commit changes with clear messages and add tests/docs.
+> 5. Keep your fork in sync with `upstream`.
+> 6. Open a pull request (PR) from your branch.
+
+---
+
+## 1) Before you start: open an issue
+Have a feature in mind? Open an issue describing the problem, proposed solution, and any alternatives. For obvious, small bug fixes, opening a PR directly is fine—but filing an issue first is still appreciated.
+
+If you’re not sure where to help, ask in an issue—documentation improvements are always welcome.
+
+## 2) Fork and clone
+
+Create your fork on GitHub, then:
+
+```bash
+git clone https://github.com/<your-username>/src.git gemc
+cd gemc
+git remote add upstream https://github.com/gemc/src.git
+git remote -v