Update README.md

Author: javadev

README.md

@@ -3,5 +3,6 @@
 [![Maven Central](https://img.shields.io/maven-central/v/com.github.javadev/leetcode-in-kotlin.svg)](http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22com.github.javadev%22%20AND%20a%3A%22leetcode-in-kotlin%22)
 [![MIT License](http://img.shields.io/badge/license-MIT-green.svg) ](https://github.com/javadev/leetcode-in-kotlin/blob/main/LICENSE)
 [![Java CI with Maven](https://github.com/javadev/LeetCode-in-Kotlin/actions/workflows/maven.yml/badge.svg)](https://github.com/javadev/LeetCode-in-Kotlin/actions/workflows/maven.yml)
+[![Maintainability Rating](https://sonarcloud.io/api/project_badges/measure?project=javadev_LeetCode-in-Kotlin&metric=sqale_rating)](https://sonarcloud.io/summary/new_code?id=javadev_LeetCode-in-Kotlin)
 
 Kotlin Solution for LeetCode algorithm problems, continually updating.