Create CONTRIBUTING.md

Author: simasaadi

CONTRIBUTING.md

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+# Contributing
+
+## Running locally
+1. `docker compose up -d`
+2. Run the queries:
+   - `docker exec -i sql_analytics_lab_db psql -U postgres -d analytics < queries/01_dimensional_metrics.sql`
+   - `docker exec -i sql_analytics_lab_db psql -U postgres -d analytics < queries/02_data_quality_checks.sql`
+   - `docker exec -i sql_analytics_lab_db psql -U postgres -d analytics < queries/03_performance_notes.sql`
+
+## Expectations
+- Keep SQL readable (CTEs, clear naming, comments for intent).
+- If you change query outputs, update the matching file in `expected_results/`.
+- CI must stay green.