[docs] Add LightGBM Tuner by Optuna links to README.md (#2893)

* Add LightGBM Tuner by Optuna links to README.md

* Removing Optuna Integrated badge

* Update README.md

Add repository code link.

* Update README.md
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

README.md

@@ -37,6 +37,7 @@ Next you may want to read:
 - [**Parameters**](https://github.com/microsoft/LightGBM/blob/master/docs/Parameters.rst) is an exhaustive list of customization you can make.
 - [**Parallel Learning**](https://github.com/microsoft/LightGBM/blob/master/docs/Parallel-Learning-Guide.rst) and [**GPU Learning**](https://github.com/microsoft/LightGBM/blob/master/docs/GPU-Tutorial.rst) can speed up computation.
 - [**Laurae++ interactive documentation**](https://sites.google.com/view/lauraepp/parameters) is a detailed guide for hyperparameters.
+- [**Optuna Hyperparameter Tuner**](https://medium.com/optuna/lightgbm-tuner-new-optuna-integration-for-hyperparameter-optimization-8b7095e99258) provides automated tuning for LightGBM hyperparameters.
 
 Documentation for contributors:
 
@@ -53,6 +54,8 @@ Some old update logs are available at [Key Events](https://github.com/microsoft/
 External (Unofficial) Repositories
 ----------------------------------
 
+Optuna (hyperparameter optimization framework): https://github.com/optuna/optuna
+
 Julia-package: https://github.com/Allardvm/LightGBM.jl
 
 JPMML (Java PMML converter): https://github.com/jpmml/jpmml-lightgbm