Add springdoc SO tag

Author: debargharoy

README.md

@@ -1,6 +1,7 @@
 [![Build Status](https://travis-ci.org/springdoc/springdoc-openapi.svg?branch=master)](https://travis-ci.org/springdoc/springdoc-openapi)
 [![Quality Gate](https://sonarcloud.io/api/project_badges/measure?project=springdoc_springdoc-openapi&metric=alert_status)](https://sonarcloud.io/dashboard?id=springdoc_springdoc-openapi)
 [![Known Vulnerabilities](https://snyk.io/test/github/springdoc/springdoc-openapi.git/badge.svg)](https://snyk.io/test/github/springdoc/springdoc-openapi.git)
+![Stack Exchange questions](https://img.shields.io/stackexchange/stackoverflow/t/springdoc)
 
 ![Octocat](https://springdoc.org/images/springdoc-openapi.png)