Add coverage badge to README

Author: tennlee

README.md

@@ -1,5 +1,7 @@
 # PyEarthTools: Reproducible science pipelines for machine learning
 
+[![Coverage Status](https://coveralls.io/repos/github/ACCESS-Community-Hub/PyEarthTools/badge.svg)](https://coveralls.io/github/ACCESS-Community-Hub/PyEarthTools)
+
 PyEarthTools is a Python framework, containing modules for loading data; pre-processing, normalising and standardising data; defining machine learning (ML) models; training ML models; performing inference with ML models; and evaluating ML models. It contains specialised support for weather and climate data sources and models. It has an emphasis on reproducibility, shareable pipelines, and human-readable low-code pipeline definition.
 
 > [!NOTE]