Add README and CONTRIBUTING documentation

[tyrsson]

Summary

Adds README.md and CONTRIBUTING.md, mirroring webware-mailer's structure with message-specific content.

Changes

• README.md: package description, quick start (composer install, middleware registration, sending + hops semantics, view helper rendering), badges.
• Badges: PHP version, latest version, license, CI, Codecov, and mutation (Stryker) — the mutation badge endpoint is wired to the 0.1.x branch and may not resolve until the Stryker dashboard works; noted separately.
• CONTRIBUTING.md: issue/PR guidance, local checks (composer scripts + mago), test conventions (PHPUnit 13 coverage metadata, mock/stub rules), coding standards.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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