docs: Add test coverage status badge

Author: alerque

README.md

@@ -2,6 +2,7 @@
 
 [![Luacheck](https://github.com/alerque/fluent-lua/workflows/Luacheck/badge.svg)](https://github.com/alerque/fluent-lua/actions)
 [![Busted](https://github.com/alerque/fluent-lua/workflows/Busted/badge.svg)](https://github.com/alerque/fluent-lua/actions)
+[![Coverage Status](https://coveralls.io/repos/github/alerque/fluent-lua/badge.svg?branch=master)](https://coveralls.io/github/alerque/fluent-lua?branch=master)
 [![GitHub tag (latest SemVer)](https://img.shields.io/github/v/tag/alerque/fluent-lua)](https://github.com/alerque/fluent-lua/releases)
 [![LuaRocks](https://img.shields.io/luarocks/v/alerque/fluent)](https://luarocks.org/modules/alerque/fluent)
 [![Join the chat at https://gitter.im/fluent-lua/community](https://badges.gitter.im/fluent-lua/community.svg)](https://gitter.im/fluent-lua/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)