ADD: badge doc to readme [no ci]

Author: 9and3

README.md

@@ -6,6 +6,7 @@
     <img src="https://github.com/diffCheckOrg/diffCheck/actions/workflows/test-pass.yml/badge.svg">
     <img src="https://github.com/diffCheckOrg/diffCheck/actions/workflows/gh-build.yml/badge.svg">
     <img src="https://github.com/diffCheckOrg/diffCheck/actions/workflows/pypi-build.yml/badge.svg">
+    <img src="https://github.com/diffCheckOrg/diffCheck/actions/workflows/doc-build.yml/badge.svg">
     <img alt="PyPI - Version" src="https://img.shields.io/pypi/v/diffCheck?style=flat&logo=pypi&logoColor=white&color=blue">
 </p>