docs: Add Platforms badge to README

- Adds Platforms badge showing macOS 13+ and iOS 16+ support
- Completes 4-badge standard: Swift, Platforms, License, CI

Author: coenttb

README.md

@@ -1,6 +1,7 @@
 # swift-emailaddress-type
 
 [![Swift](https://img.shields.io/badge/Swift-6.0-orange.svg)](https://swift.org)
+[![Platforms](https://img.shields.io/badge/Platforms-macOS%2013%2B%20|%20iOS%2016%2B-lightgray.svg)]()
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
 [![CI](https://github.com/coenttb/swift-emailaddress-type/workflows/CI/badge.svg)](https://github.com/coenttb/swift-emailaddress-type/actions/workflows/ci.yml)