Add codecov badge

Author: westonruter

README.md

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 # Performance Lab
 ![Performance Lab plugin banner with icon](https://github.com/WordPress/performance/assets/10103365/99d37ba5-27e3-47ea-8ab8-48de75ee69bf)
 
+[![codecov](https://codecov.io/gh/WordPress/performance/graph/badge.svg?token=zQv52K2LWz)](https://codecov.io/gh/WordPress/performance)
+
 Monorepo for the [WordPress Performance Team](https://make.wordpress.org/performance/), primarily for the Performance Lab plugin, which is a collection of standalone performance features.
 
 Details about the Performance Lab plugin, including instructions for getting started and contributing, are available in the [Performance Team Handbook here](https://make.wordpress.org/performance/handbook/performance-lab/).