Add contributing guide

[MichielDean]

Summary

Introduces CONTRIBUTING.md — a guide documenting how to set up the project, build, test, and submit changes to FreeFlow. Also links it from the README and adds a changelog entry.

Motivation

FreeFlow has no documented contribution process today — no CONTRIBUTINGING.md, no issue/PR templates, no contributor guide. New contributors have to reverse-engineer the process from merged PRs and the Makefile. This lowers the barrier for community participation.

The guide reflects the conventions already in use across the 108 merged PRs: fork-and-PR workflow, make / make test for building, focused PRs, changelog entries under [Unreleased], and responsive engagement with the CodeRabbit review bot. It does not invent new rules.

Behavior

No code or runtime behavior changes. Adds a documentation file, a README link, and a changelog entry.

Changes

• CONTRIBUTING.md (new) — Covers prerequisites (macOS 13+, Xcode CLI tools, Groq key), getting the source, building (make, make run, make ARCH=universal), testing (make test, how to extend the test runner), development conventions (branch naming, commit messages, code style, changelog format), submitting changes (PR description structure, size guidance, CodeRabbit review), reporting issues, and discussions.
• README.md (+4) — Adds a ## Contributing section linking to CONTRIBUTING.md.
• CHANGELOG.md (+6) — Adds an [Unreleased] → Added entry for the guide.

Test plan

• CONTRIBUTING.md content reflects actual repo conventions (verified against Makefile targets, existing PR patterns, CHANGELOG format, and test runner structure).
• README link resolves to the new file.
• Changelog entry follows the existing style (sentence case, no trailing period).
• No code changes — make / make test unaffected.

Notes

If the maintainers prefer a different structure or scope (e.g., shorter, split into separate files, or covering additional topics like release process or security reporting), happy to adjust.

Summary by CodeRabbit

• Documentation
  • Added a contributor guide covering setup, development, testing, coding conventions, changelog updates, pull requests, issue reporting, and licensing.
  • Added a README link to the new contribution guidelines.
  • Documented the contribution guide in the unreleased changelog.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 4f583e46-8c94-48b5-b57d-30cbfefb7d58

📥 Commits

Reviewing files that changed from the base of the PR and between 02e000a and 68b8d56.

📒 Files selected for processing (2)

• CHANGELOG.md
• CONTRIBUTING.md

🚧 Files skipped from review as they are similar to previous changes (2)

• CHANGELOG.md
• CONTRIBUTING.md

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📝 Walkthrough

Walkthrough

Added CONTRIBUTING.md with setup, build, test, workflow, and submission instructions. Linked the guide from README.md and recorded it in the [Unreleased] section of CHANGELOG.md.

Changes

Contribution documentation

Layer / File(s)	Summary
Contribution guide setup and validation CONTRIBUTING.md	Added prerequisites, repository setup, build commands, test commands, and test coverage guidance.
Contribution workflow conventions CONTRIBUTING.md	Added branch, commit, coding style, changelog, pull request, issue reporting, discussion, and licensing conventions.
Contribution guide repository links README.md, CHANGELOG.md	Linked the contribution guide from the README and documented it in the unreleased changelog.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding a contributing guide.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@CONTRIBUTING.md`:
- Line 3: Update the introductory description in CONTRIBUTING.md to use the
hyphenated compound modifier “open-source” in the phrase describing FreeFlow,
leaving the surrounding wording unchanged.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ed705860-179c-429e-9381-b95ff9e0850a

📥 Commits

Reviewing files that changed from the base of the PR and between e63a232 and 02e000a.

📒 Files selected for processing (3)

• CHANGELOG.md
• CONTRIBUTING.md
• README.md