Add documentation badge in README

Author: kelson42

README.md

@@ -8,6 +8,7 @@ GNU/Linux, macOS, Android, iOS, ...).
 [![Release](https://img.shields.io/github/v/tag/kiwix/libkiwix?label=release&sort=semver)](https://download.kiwix.org/release/libkiwix/)
 [![Repositories](https://img.shields.io/repology/repositories/libkiwix?label=repositories)](https://github.com/kiwix/libkiwix/wiki/Repology)
 [![Build Status](https://github.com/kiwix/libkiwix/workflows/CI/badge.svg?query=branch%3Amaster)](https://github.com/kiwix/libkiwix/actions?query=branch%3Amaster)
+[![Doc](https://readthedocs.org/projects/libkiwix/badge/?style=flat)](https://libkiwix.readthedocs.org/en/latest/?badge=latest)
 [![CodeFactor](https://www.codefactor.io/repository/github/kiwix/libkiwix/badge)](https://www.codefactor.io/repository/github/kiwix/libkiwix)
 [![Codecov](https://codecov.io/gh/kiwix/libkiwix/branch/master/graph/badge.svg)](https://codecov.io/gh/kiwix/libkiwix)
 [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)