Merge pull request #8537 from chainer/Crissman-optuna

Add Optuna to README.md

README.md

@@ -7,6 +7,7 @@
 [![travis](https://img.shields.io/travis/chainer/chainer/master.svg)](https://travis-ci.org/chainer/chainer)
 [![coveralls](https://img.shields.io/coveralls/chainer/chainer.svg)](https://coveralls.io/github/chainer/chainer)
 [![Read the Docs](https://readthedocs.org/projects/chainer/badge/?version=stable)](https://docs.chainer.org/en/stable/?badge=stable)
+[![Optuna](https://img.shields.io/badge/Optuna-integrated-blue)](https://optuna.org)
 
 [**Website**](https://chainer.org/)
 | [**Docs**](https://docs.chainer.org/en/stable/)