Add code coverage badge to README.md

Author: johnzl-777

README.md

@@ -1,6 +1,7 @@
 # ParallelMergeCSR.jl
 
 [![Build Status](https://github.com/QuEraComputing/Bloqade.jl/workflows/CI/badge.svg)](https://github.com/QuEraComputing/Bloqade.jl/actions)
+[![codecov](https://codecov.io/gh/QuEraComputing/ParallelMergeCSR.jl/branch/main/graph/badge.svg?token=P0UCC5CAVB)](https://codecov.io/gh/QuEraComputing/ParallelMergeCSR.jl)
 
 <p>
 An implementation/port of <a href="https://rd.yyrcd.com/CUDA/2022-03-14-Merge-based%20Parallel%20Sparse%20Matrix-Vector%20Multiplication.pdf"> Merrill and Garland's Merge-based Parallel Sparse Matrix-Vector Multiplication (10.1109/SC.2016.57) </a> paper in