add readthedocs badge

README.md

@@ -9,6 +9,7 @@
 [![MacOS](https://github.com/yoshida-lab/XenonPy/workflows/MacOS/badge.svg)](https://github.com/yoshida-lab/XenonPy/actions?query=workflow%3AMacOS)
 [![Windows](https://github.com/yoshida-lab/XenonPy/workflows/Windows/badge.svg)](https://github.com/yoshida-lab/XenonPy/actions?query=workflow%3AWindows)
 [![Ubuntu](https://github.com/yoshida-lab/XenonPy/workflows/Ubuntu/badge.svg)](https://github.com/yoshida-lab/XenonPy/actions?query=workflow%3AUbuntu)
+[![Documentation Status](https://readthedocs.org/projects/xenonpy/badge/?version=latest)](https://xenonpy.readthedocs.io/en/latest/?badge=latest)
 [![codecov](https://codecov.io/gh/yoshida-lab/XenonPy/branch/master/graph/badge.svg)](https://codecov.io/gh/yoshida-lab/XenonPy)
 [![Version](https://img.shields.io/github/tag/yoshida-lab/XenonPy.svg?maxAge=360)](https://github.com/yoshida-lab/XenonPy/releases/latest)
 [![Python Versions](https://img.shields.io/pypi/pyversions/xenonpy.svg)](https://pypi.org/project/xenonpy/)

docs/source/index.rst

@@ -8,7 +8,7 @@
 What is XenonPy project
 ========================
 
-|MacOS| |Windows| |Ubuntu| |codecov| |version| |python| |total-dl| |per-dl|
+|MacOS| |Windows| |Ubuntu| |readthedocs| |codecov| |version| |python| |total-dl| |per-dl|
 
 --------
 Overview
@@ -97,6 +97,10 @@ Code contributions are also very welcomed. See :doc:`contribution` for more deta
     :alt: Ubuntu Building Status
     :target: https://github.com/yoshida-lab/XenonPy/actions?query=workflow%3AUbuntu
 
+.. |readthedocs| image:: https://readthedocs.org/projects/xenonpy/badge/?version=latest
+    :alt: Documentation Status
+    :target: https://xenonpy.readthedocs.io/en/latest/?badge=latest
+
 .. |codecov| image:: https://codecov.io/gh/yoshida-lab/XenonPy/branch/master/graph/badge.svg
   :target: https://codecov.io/gh/yoshida-lab/XenonPy