docs: add contributing guide

Author: edouard-claude

CONTRIBUTING.md

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+# Contributing to snip
+
+Thanks for your interest! Here's how to get started.
+
+## Quick Start
+
+```bash
+git clone https://github.com/edouard-claude/snip.git
+cd snip
+make build
+make test
+```
+
+## Adding a Filter
+
+Filters are YAML files in `filters/`. No Go knowledge required.
+
+1. Create `filters/your-tool.yaml` following existing filters as examples
+2. Add test fixtures in `tests/fixtures/`
+3. Run `make test`
+4. Open a PR
+
+See the [Filter DSL documentation](https://github.com/edouard-claude/snip/wiki) for available actions.
+
+## Code Changes
+
+1. Fork and create a feature branch
+2. Write tests first (TDD)
+3. Run `make test && make lint`
+4. Commit with [conventional prefixes](https://www.conventionalcommits.org/): `fix:`, `feat:`, `docs:`, `ci:`, `test:`
+5. Open a PR against `master`
+
+## Guidelines
+
+- Keep PRs focused — one fix or feature per PR
+- All code, comments, and commits in English
+- Generated files (`db/`, `gql/generated.go`) should not be edited manually
+- Startup performance matters — snip intercepts every shell command
+
+## Questions?
+
+Open a [discussion](https://github.com/edouard-claude/snip/issues) or check existing issues.