Create contributing.md (#922)

Author: sbryngelson

.github/CONTRIBUTING.md

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+# Contributing to the MFC Codebase (Multi‑Component Flow Code)
+
+**Multi‑Component Flow Code (MFC)** is an open‑source, high‑performance code for simulating compressible multi‑component, multi‑phase flows.
+We welcome contributions of all kinds—bug fixes, new features, documentation, tests, and issue triage—from both newcomers and experienced developers.
+This guide explains how to set up your environment, follow MFC's coding standards, and navigate the pull-request (PR) process so your work can be merged smoothly.
+
+---
+
+## 1. Setting Up Your Development Environment
+
+1. **Fork and clone**
+   ```bash
+   git clone https://github.com/<your‑user>/MFC.git
+   cd MFC
+   git remote add upstream https://github.com/MFlowCode/MFC.git
+   ```
+2. **Build MFC** – follow the [documentation](https://mflowcode.github.io/documentation/md_getting-started.html). For example:
+   ```bash
+   ./mfc.sh build -j 8   # parallel build with 8 threads
+   ```
+3. **Run the test suite** to verify your environment:
+   ```bash
+   ./mfc.sh test -j 8
+   ```
+
+---
+
+## 2. Development Workflow
+
+| Step | Action | Notes |
+|------|--------|-------|
+| 1 | **Sync your fork**: `git checkout master && git pull upstream master` | Stay up‑to‑date to avoid merge conflicts. |
+| 2 | **Create a branch**: `git checkout -b feature/<short‑name>` | Keep each branch focused on one logical change. |
+| 3 | **Code, test, document** | Follow the guidelines in §3. |
+| 4 | **Format & lint**: `./mfc.sh format` | CI will re‑check; make it pass locally first. |
+| 5 | **Run tests**: `./mfc.sh test` | All existing and new tests must pass. |
+| 6 | **Commit** (see *Commit Messages* below) | Write clear, atomic commits. |
+| 7 | **Push** & open a **PR** | Be mindful: *every push triggers CI*. Bundle fixes together to avoid dozens of CI runs. |
+
+### Commit Messages
+
+- Start with a concise (≤50 chars) summary in imperative mood: `Fix out‑of‑bounds in EOS module`.
+- Add a blank line, then a detailed explanation.
+- Reference related issues or PRs, e.g., `Fixes #123`.
+
+### Managing CI Runs
+
+Each push to a branch with an open PR runs the full CI matrix (which can take hours).
+Plan your pushes—run tests locally and group changes—so the CI queue is not flooded.
+
+---
+
+## 3. Coding Guidelines and Best Practices
+
+### 3.1 Style, Formatting & Linting
+MFC enforces a project‑wide Fortran style:
+- **Formatter**: `./mfc.sh format` auto‑formats your changes.
+- **Linter**: CI runs several linter checks that spot common Fortran-gotchas (implicit typing, shadowed variables, unused locals, etc.). Fix issues before pushing or the linter will often catch them.
+
+### 3.2 Fypp Metaprogramming
+
+MFC uses [**Fypp**](https://github.com/aradi/fypp), a lightweight Python-based preprocessor, to generate repetitive or accelerator-specific Fortran.
+Key points:
+- Fypp macros live in `include/` directories nested within `src/`.
+- Run `./mfc.sh format` to format the example case files and the source code.
+- When editing `.fpp`, maintain readability, prefer simple macros over deeply nested constructs.
+
+### 3.3 Documentation
+
+- Add or update Doxygen comments in source files.
+- Update Markdown docs under `docs/` if user‑facing behavior changes.
+- Provide a minimal example in `examples/` for each new feature when practical.
+
+### 3.4 Testing
+
+- Add regression tests that fail before your change and pass after.
+- Use `./mfc.sh test --generate` to create golden files for new cases.
+- Keep tests fast; favor small grids and short runtimes.
+
+### 3.5 GPU & Performance
+
+- Ensure code compiles for CPU *and* GPU targets (NVHPC for NVIDIA, Cray for AMD).
+- Profile critical kernels; avoid introducing bottlenecks.
+
+---
+
+## 4. Preparing Your Pull Request
+
+1. **One PR = One logical change**. If you plan a follow‑up change, open an issue describing it and assign yourself for visibility.
+2. **Fill out the PR template**. Remove checkboxes that do **not** apply.
+3. **Describe testing** – list commands, compilers, and any profiling.
+4. **Link issues** – `Fixes #<id>` or `Part of #<id>`.
+5. **Ensure CI passes** before requesting review.
+
+> **Tip** If your change is large, consider splitting it into smaller PRs. Document the intent in an issue so reviewers understand the overall roadmap.
+
+---
+
+## 5. Code Review & Merge
+
+- Respond promptly to reviewer comments.
+- Push focused updates; each push re‑runs CI.
+- When all reviews are approved and CI is green, a maintainer will merge your PR.
+
+---
+
+## 6. Issue Triage
+
+If you prefer helping with issue management:
+- Comment to clarify reproduction steps.
+- Label issues when you have triage rights.
+- Close fixed issues and reference the PR.
+