Update Readme to include Cornac-AB section

[darrylong]

Description

This PR adds a section in the readme to showcase an AB-testing framework which uses the Cornac API serving extension.

Related Issues

Checklist:

• [ ] I have added tests.
• [ ] I have updated the documentation accordingly.
• [ ] I have updated README.md (if you are adding a new model).
• [ ] I have updated examples/README.md (if you are adding a new example).
• [ ] I have updated datasets/README.md (if you are adding a new dataset).