Pull Requests creation and Branches management

@Yonv1943 I noticed you open two new branches to create [Pull Requests 218](https://github.com/AI4Finance-Foundation/ElegantRL/tree/217-add-unit-tests-for-elegantrl), and [Pull Request 216](https://github.com/AI4Finance-Foundation/ElegantRL/tree/213-selfstates0). The number of branches plays a key role in open-source projects. We should discuss how to create Pull Requests and how to manage Branches.