docs: improve README introduction and iOS testing guidance

[dcavalcante]

Summary

• rewrite the README introduction to explain what the library is and why Wi-Fi Aware is useful
• add focused instructions for developers who can validate the iOS 26 implementation on physical hardware
• add a structured iOS validation issue form for physical-device reports
• document how contributors should report Apple validation results safely and reproducibly
• clarify the current iOS/iPadOS and Android-to-iOS validation goals without changing the implementation status

Scope

Docs and contribution scaffolding only. This is intentionally separate from #4 so the iOS lifecycle fix remains focused and easy to review.