docs: add CONTRIBUTING.md for contributors

CONTRIBUTING.md

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+# Contributing to sqlx
+
+Thank you for your interest in contributing to sqlx.
+
+## Before you open an issue or PR
+
+- Search [existing issues](https://github.com/jmoiron/sqlx/issues) and [pull requests](https://github.com/jmoiron/sqlx/pulls) first.
+- For bugs, include a minimal Go program that reproduces the problem, your Go version, and database driver if relevant.
+- sqlx extends `database/sql`; confirm the behavior with plain `database/sql` when possible to isolate driver-specific issues.
+
+## Pull requests
+
+- Keep changes focused; avoid unrelated formatting or refactors.
+- Add or update tests for behavior changes (`go test ./...`).
+- Match existing code style in the files you touch.
+
+## Development
+
+```bash
+go test ./...
+```
+
+Some tests require database drivers; run only package tests if you do not have databases configured locally.
+
+## License
+
+By contributing, you agree that your contributions are licensed under the same terms as the project (MIT).