Add Codecov badge to README

Author: ndbroadbent

README.md

@@ -4,6 +4,7 @@ A thread-safe, generic ordered map for Go that maintains insertion order while p
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/DocSpring/orderedmap.svg)](https://pkg.go.dev/github.com/DocSpring/orderedmap)
 [![Go Report Card](https://goreportcard.com/badge/github.com/DocSpring/orderedmap)](https://goreportcard.com/report/github.com/DocSpring/orderedmap)
+[![codecov](https://codecov.io/gh/DocSpring/orderedmap/branch/main/graph/badge.svg)](https://codecov.io/gh/DocSpring/orderedmap)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
 ## Features