docs: add CONTRIBUTING.md development guide

CONTRIBUTING.md

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+# Contributing to gpuaudit
+
+Thanks for helping improve gpuaudit.
+
+## Build and test
+
+```bash
+git clone https://github.com/gpuaudit/cli.git
+cd cli
+go build -o gpuaudit ./cmd/gpuaudit
+go test ./...
+```
+
+## Project layout
+
+See [README.md](README.md) for architecture and scanner overview. Core packages live under `internal/` (analysis rules, cloud providers, output formatters, and the CLI in `cmd/gpuaudit`).
+
+## Adding an analysis rule
+
+1. Implement the rule in `internal/analysis/rules.go` (or a focused helper called from `analyzeInstance`).
+2. Register the rule in the `analyzeInstance` flow so matching instances get recommendations.
+3. Add tests under `internal/analysis/` covering the new signal.
+
+## Adding an output format
+
+1. Implement a formatter under `internal/output/`.
+2. Wire the format into the CLI flag handling in `cmd/gpuaudit`.
+3. Add tests with golden or snapshot output where practical.
+
+## Adding a cloud provider
+
+1. Add provider-specific scanning under `internal/cloud/` (follow existing AWS patterns).
+2. Integrate the scanner into the main `scan` pipeline.
+3. Document required credentials and flags in the README.
+
+## Pull requests
+
+- One feature or fix per PR
+- Include tests for behavior changes
+- Describe how you validated the change (`go test ./...`, sample scan output, etc.)