docs: add CONTRIBUTING.md to guide new contributors on project participation

Author: pmoscode

CONTRIBUTING.md

@@ -0,0 +1,53 @@
+# Contributing to go-common
+
+Thank you for considering contributing to **go-common**! Every contribution is welcome, whether it's a bug report, a feature request, or a pull request.
+
+## How to Report a Bug
+
+1. Check the [existing issues](https://github.com/pmoscode/go-common/issues) to see if the bug has already been reported.
+2. If not, open a new issue with:
+   - A clear and descriptive title
+   - Steps to reproduce the problem
+   - Expected vs. actual behavior
+   - Go version and OS
+
+## How to Suggest a Feature
+
+Open an issue with the **feature request** label. Describe the use case and why the feature would be useful.
+
+## How to Submit a Pull Request
+
+1. **Fork** the repository and create a new branch from `main`.
+2. **Install dependencies:**
+   ```bash
+   go mod tidy
+   ```
+3. **Make your changes.** Follow the conventions below.
+4. **Run the tests:**
+   ```bash
+   go test ./...
+   ```
+5. **Run static analysis** (if [staticcheck](https://staticcheck.dev/docs/getting-started/) is installed):
+   ```bash
+   staticcheck ./...
+   ```
+6. **Commit** with a clear message describing what you changed and why.
+7. **Open a Pull Request** against `main`.
+
+## Code Conventions
+
+- **File naming:** Name the primary file in a package after the package itself (e.g. `filter/filter.go`), not `main.go`.
+- **Error handling:** Return errors to the caller. Do not use `panic`, `log.Fatal`, or `fmt.Println` for error handling in library code.
+- **Logging:** Use the `log` standard library for warnings. Do not use `fmt.Print*` for log output.
+- **GoDoc:** All exported types, functions, and methods must have GoDoc comments.
+- **Deprecation:** Use the standard Go `// Deprecated: ...` comment format.
+- **Testing:** Use [testify](https://github.com/stretchr/testify) (`assert` / `require`) for assertions. Use `require` for setup errors and `assert` for value checks.
+
+## Security
+
+If you discover a security vulnerability, please **do not** open a public issue. Instead, refer to the [Security Policy](SECURITY.md) for instructions on responsible disclosure.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the same license as the project (see [LICENSE](LICENSE)).
+