Add CONTRIBUTING guide (#48)

* docs: add contributing guide

Fixes #47.

* Update CONTRIBUTING.md

---------

Co-authored-by: alexx-ftw <alexx-ftw@users.noreply.github.com>
Co-authored-by: Game_Time <108236317+RayBytes@users.noreply.github.com>

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# Contributing to ChatMock
+
+We welcome thoughtful improvements. This guide calls out the expectations that keep reviews quick and the project stable.
+
+# How should I contribute?
+
+### Before changing code...
+- Open an issue before large or risky efforts so scope is agreed up front.
+- Keep pull requests focused and easy to follow & break sweeping changes into a series when possible.
+- Treat documentation, code, and packaging (CLI, Docker, GUI) as a single surface (your updates should apply to all).
+
+### Getting Set Up
+- Review the Quickstart section in README.md
+- Go through the codebase, and ensure you understand the current codebase. 
+- Confirm you can log in and serve a local instance, then make a couple of sample requests to understand current behaviour so you know if it broke later on.
+
+### Working With Core Files
+- `prompt.md` and related Codex harness files are sensitive. Do not modify them or move entry points without prior maintainer approval.
+- Be cautious with parameter names, response payload shapes, and file locations consumed by downstream clients. Coordinate before changing them.
+- When touching shared logic, update both OpenAI and Ollama routes, plus any CLI/GUI code that depends on the same behaviour.
+
+## Designing Features and Fixes
+- Prefer opt-in flags or config switches for new capabilities & leave defaults unchanged until maintainers confirm the rollout plan.
+- Document any limits, or external dependencies introduced by your change.
+- Validate compatibility with popular clients (e.g. Jan, Raycast, custom OpenAI SDKs) when responses or streaming formats shift.
+
+# Pull Request Checklist
+- [ ] Rebased on the latest `main` and issue reference included when applicable.
+- [ ] Manual verification steps captured under "How to try locally" in the PR body.
+- [ ] README.md, DOCKER.md, and other docs updated—or explicitly noted as not required.
+- [ ] No generated artefacts or caches staged (`build/`, `dist/`, `__pycache__/`, `.pytest_cache/`, etc.).
+- [ ] Critical paths (`prompt.md`, routing modules, public parameter names) reviewed for unintended edits and discussed with maintainers if changes were necessary.
+
+## Need Help?
+- If you're not sure about about scope, flags, or how to implement a certain feature, always create an issue before hand.
+
+Thank you for you contribution!