[docs] Add new badges (#3610)

* Update README.md

* Update README.md

* Update README.rst

R-package/README.md

 # LightGBM R-package
 
+[![CRAN Version](https://www.r-pkg.org/badges/version/lightgbm)](https://cran.r-project.org/package=lightgbm)
+[![Downloads](https://cranlogs.r-pkg.org/badges/grand-total/lightgbm)](https://cran.r-project.org/package=lightgbm)
+[![API Docs](https://readthedocs.org/projects/lightgbm/badge/?version=latest)](https://lightgbm.readthedocs.io/en/latest/R/reference/)
+
 <img src="man/figures/logo.svg" align="right" alt="" width="175" />
 
 ### Contents

README.md

@@ -11,6 +11,7 @@ Light Gradient Boosting Machine
 [![License](https://img.shields.io/github/license/microsoft/lightgbm.svg)](https://github.com/microsoft/LightGBM/blob/master/LICENSE)
 [![Python Versions](https://img.shields.io/pypi/pyversions/lightgbm.svg?logo=python&logoColor=white)](https://pypi.org/project/lightgbm)
 [![PyPI Version](https://img.shields.io/pypi/v/lightgbm.svg?logo=pypi&logoColor=white)](https://pypi.org/project/lightgbm)
+[![CRAN Version](https://www.r-pkg.org/badges/version/lightgbm)](https://cran.r-project.org/package=lightgbm)
 [![Join Gitter at https://gitter.im/Microsoft/LightGBM](https://badges.gitter.im/Microsoft/LightGBM.svg)](https://gitter.im/Microsoft/LightGBM?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Slack](https://lightgbm-slack-autojoin.herokuapp.com/badge.svg)](https://lightgbm-slack-autojoin.herokuapp.com)
 

python-package/README.rst

 LightGBM Python-package
 =======================
 
-|License| |Python Versions| |PyPI Version| |Downloads|
+|License| |Python Versions| |PyPI Version| |Downloads| |API Docs|
 
 Installation
 ------------
@@ -205,4 +205,6 @@ Documentation strings (docstrings) are written in the NumPy style.
    :target: https://pypi.org/project/lightgbm
 .. |Downloads| image:: https://pepy.tech/badge/lightgbm
    :target: https://pepy.tech/project/lightgbm
+.. |API Docs| image:: https://readthedocs.org/projects/lightgbm/badge/?version=latest
+   :target: https://lightgbm.readthedocs.io/en/latest/Python-API.html
 .. _CMake: https://cmake.org/