docs: add codecov badge

Continuation of #5098.

Author: vil02

README.md

@@ -1,6 +1,7 @@
 # The Algorithms - Java
 
 [![Build](https://github.com/TheAlgorithms/Java/actions/workflows/build.yml/badge.svg?branch=master)](https://github.com/TheAlgorithms/Java/actions/workflows/build.yml)
+[![codecov](https://codecov.io/gh/TheAlgorithms/Java/graph/badge.svg?token=XAdPyqTIqR)](https://codecov.io/gh/TheAlgorithms/Java)
 [![Discord chat](https://img.shields.io/discord/808045925556682782.svg?logo=discord&colorB=7289DA&style=flat-square)](https://discord.gg/c7MnfGFGa6)
 [![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/TheAlgorithms/Java)