Update README.md

Author: GiggleLiu

README.md

@@ -1,6 +1,7 @@
 # GraphTensorNetworks
 
 [![Build Status](https://github.com/Happy-Diode/GraphTensorNetworks.jl/workflows/CI/badge.svg)](https://github.com/Happy-Diode/GraphTensorNetworks.jl/actions)
+[![Coverage Status](https://coveralls.io/repos/github/Happy-Diode/GraphTensorNetworks.jl/badge.svg?branch=fix-test-coverage&t=rIJIK2)](https://coveralls.io/github/Happy-Diode/GraphTensorNetworks.jl?branch=fix-test-coverage)
 
 ## Installation
 <p>