Add Codecov badge to README (#30)

Author: slashmo

README.md

@@ -1,6 +1,7 @@
 # Swift W3C TraceContext
 
 [![CI](https://github.com/swift-otel/swift-w3c-trace-context/actions/workflows/ci.yml/badge.svg)](https://github.com/swift-otel/swift-w3c-trace-context/actions/workflows/ci.yml)
+[![codecov](https://codecov.io/gh/swift-otel/swift-w3c-trace-context/graph/badge.svg?token=9656QtdhYA)](https://codecov.io/gh/swift-otel/swift-w3c-trace-context)
 [![Swift](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fswift-otel%2Fswift-w3c-trace-context%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/swift-otel/swift-w3c-trace-context)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fswift-otel%2Fswift-w3c-trace-context%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/swift-otel/swift-w3c-trace-context)