DOC: add CONTRIBUTING.md adapted from DataLad

CONTRIBUTING.md

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+# Contributing to expfactory-deploy
+
+Thanks for your interest in contributing! This project deploys Django-managed batteries of experiments. If you hit a snag or have a question, please open an issue at <https://github.com/expfactory/expfactory-deploy/issues>.
+
+## Attribution
+
+The structure and conventions of this document (branch prefixes, commit prefixes, fork + PR workflow) are adapted from the excellent [DataLad CONTRIBUTING guide](https://github.com/datalad/datalad/blob/maint/CONTRIBUTING.md). Thanks to the DataLad maintainers.
+
+## Development setup
+
+See `README.rst` for full local setup. In short: the project runs as a Docker Compose stack defined in `local.yml`; you will need a container runtime (Docker or Podman) and a few env files under `.envs/.local/`.
+
+## Fork-based workflow
+
+1. Fork `expfactory/expfactory-deploy` on GitHub.
+2. Clone your fork locally as `origin`:
+   ```bash
+   git clone git@github.com:<your-username>/expfactory-deploy.git
+   cd expfactory-deploy
+   ```
+3. Add the canonical repository as `upstream`:
+   ```bash
+   git remote add upstream https://github.com/expfactory/expfactory-deploy.git
+   ```
+4. Keep `main` tracking `upstream/main`:
+   ```bash
+   git fetch upstream
+   git switch main
+   git branch --set-upstream-to=upstream/main main
+   ```
+5. Create topic branches off `main`. See **Branch naming** below.
+6. Push topic branches to your fork (`origin`) and open a PR against `expfactory/expfactory-deploy:main`.
+
+## Branch naming
+
+Use these prefixes on topic-branch names, lowercase and hyphen-separated:
+
+| Prefix | Meaning          |
+| ------ | ---------------- |
+| `nf-`  | New feature      |
+| `bf-`  | Bug fix          |
+| `rf-`  | Refactor         |
+| `doc-` | Documentation    |
+| `bm-`  | Benchmarks       |
+
+Examples: `nf-prolific-export`, `bf-preview-instructions-typo`, `doc-contributing`.
+
+## Commit messages
+
+Prefix the first line of each commit with one of these tags:
+
+| Prefix | Meaning                 |
+| ------ | ----------------------- |
+| `NF:`  | New feature             |
+| `BF:`  | Bug fix                 |
+| `RF:`  | Refactor                |
+| `DOC:` | Documentation           |
+| `BM:`  | Benchmarks              |
+| `TST:` | Tests only              |
+| `CI:`  | Continuous integration  |
+| `UX:`  | User-facing polish      |
+| `BK:`  | Known breakage          |
+
+Prefixes may be combined with `+`, e.g. `RF+DOC: rename battery model and update docs`.
+
+You may scope a commit with parentheses, e.g. `BF(TST): fix flaky assertion in prolific view test`.
+
+To auto-close an issue when the PR merges, add a footer: `Closes #123`.
+
+## Before opening a PR
+
+- [ ] `main` is up to date with `upstream/main`, and your topic branch is rebased or merged up to that tip.
+- [ ] Pre-commit hooks pass (the repo has `.pre-commit-config.yaml` configured with ruff and other checks).
+- [ ] Database migrations are included if you changed models.
+- [ ] Tests pass locally:
+  ```bash
+  podman compose -f local.yml run --rm django ./manage.py test
+  # or: docker compose -f local.yml run --rm django ./manage.py test
+  ```
+
+## Opening the PR
+
+- **Base:** `expfactory/expfactory-deploy:main`.
+- **Head:** `<your-username>:<branch-name>`.
+- **Title:** start with the commit prefix that matches the dominant change (e.g., `BF: fix ...`).
+- **Body:** short description of what and why; link any related issues.
+
+## After merge
+
+Once your PR merges:
+
+1. Delete the topic branch from your fork (GitHub's UI offers a one-click delete after merge).
+2. Sync your fork's `main`:
+   ```bash
+   git switch main
+   git fetch upstream
+   git reset --hard upstream/main
+   git push --force-with-lease origin main
+   ```