Add Codecov badge to README

Added Codecov badge to README for coverage tracking.

Author: vincentlaucsb

README.md

@@ -1,5 +1,5 @@
 # Vince's CSV Parser
-[![CMake on Windows](https://github.com/vincentlaucsb/csv-parser/actions/workflows/cmake-multi-platform.yml/badge.svg)](https://github.com/vincentlaucsb/csv-parser/actions/workflows/cmake-multi-platform.yml) [![Memory and Thread Sanitizers](https://github.com/vincentlaucsb/csv-parser/actions/workflows/sanitizers.yml/badge.svg)](https://github.com/vincentlaucsb/csv-parser/actions/workflows/sanitizers.yml)
+[![CMake on Windows](https://github.com/vincentlaucsb/csv-parser/actions/workflows/cmake-multi-platform.yml/badge.svg)](https://github.com/vincentlaucsb/csv-parser/actions/workflows/cmake-multi-platform.yml) [![Memory and Thread Sanitizers](https://github.com/vincentlaucsb/csv-parser/actions/workflows/sanitizers.yml/badge.svg)](https://github.com/vincentlaucsb/csv-parser/actions/workflows/sanitizers.yml) [![codecov](https://codecov.io/gh/vincentlaucsb/csv-parser/graph/badge.svg?token=8gJtCOfP3d)](https://codecov.io/gh/vincentlaucsb/csv-parser)
 
 - [Vince's CSV Parser](#vinces-csv-parser)
   - [Motivation](#motivation)