docs(coverage): add coverage badge

Author: Xenira

README.md

@@ -4,6 +4,7 @@
 [![docs.rs](https://img.shields.io/docsrs/ext-php-rs/latest)](https://docs.rs/ext-php-rs)
 [![Guide Workflow Status](https://img.shields.io/github/actions/workflow/status/davidcole1340/ext-php-rs/docs.yml?branch=master&label=guide)](https://davidcole1340.github.io/ext-php-rs)
 ![CI Workflow Status](https://img.shields.io/github/actions/workflow/status/davidcole1340/ext-php-rs/build.yml?branch=master)
+[![Coverage Status](https://coveralls.io/repos/github/davidcole1340/ext-php-rs/badge.svg)](https://coveralls.io/github/davidcole1340/ext-php-rs)
 [![Discord](https://img.shields.io/discord/115233111977099271)](https://discord.gg/dphp)
 
 Bindings and abstractions for the Zend API to build PHP extensions natively in