Improve README based on feedback

[JohnGiorgi]

There are several small changes to the README that should be made based on internal feedback:

• [ ] Not clear which versions of python are supported. Add this line right under installation and before the code snippet: This repository requires Python 3.7 or later
• [ ] ktrain throws a UserWarning which may be confusing to a user. There are two options here: See if ktrain has an option for disabling user warnings and use it, or, add a comment under the code snippet in quickstart: ktrain may throw a UserWarning which you can safely ignore.

[JohnGiorgi]

This was closed by #13. @Steven-Palayew If you want the action of merging a PR to automatically close an issue, use any of the closing keywords (https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) in your PR comment. I just go with "Closes #12".