Add coverage badge

README.md

@@ -1,5 +1,6 @@
 [![Go Reference](https://pkg.go.dev/badge/github.com/unitoftime/flow.svg)](https://pkg.go.dev/github.com/unitoftime/flow)
 [![build](https://github.com/unitoftime/flow/actions/workflows/build.yml/badge.svg)](https://github.com/unitoftime/flow/actions/workflows/build.yml)
+[![Go Coverage](https://github.com/unitoftime/flow/wiki/coverage.svg)](https://raw.githack.com/wiki/unitoftime/flow/coverage.html)
 
 # Overview
 Flow is a work in progress component library that I use in my games and other applications. You're welcome to check it out. Each package inside here represents a different domain