OSS-561 - Rasa Interactive Learning

[antoniotoineto]

Proposed changes:

• Add a Interactive Learning session at Documentation. I've added to the ReadMe because I can't add it to the official docs which is ideal.
• Solve some doubts that were related at issue OSS-561 by adding this IL session explicitly in the ReadMe, that would help people to know better Rasa Interactive Learning

Status (please check what you already did):

• [ ] added some tests for the functionality
• [x] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)