Create README.md file

Problem/Motivation

Module requires documentation in order to explain to developers what this module is for and how to use it.
#2909484: How to test the webhook with an API.AI agent is a symptom of this requirement.

Proposed resolution

Create a README.md file for initial documentation. The file should contain:
- A brief introduction to the module and Api.AI
- Description of the webhook system
- Authentication options
- How to use it with Chatbot API intents and entities push.

Remaining tasks

- Working on a patch
- Testing/Reviewing
- Commit

User interface changes

No.

API changes

No.

Data model changes

No.