Add documentation for API endpoints

[prayujt]

Problem To Be Solved

There is no documentation for the HTTP endpoints

Motivation

This will make it very difficult for the front-end team to understand what to pass in to each request and what they will receive back, and requires them to look through the back-end code and figure it out themselves.

Proposed Solution

Add a README.md file in the api folder, or some other form of documentation that records each endpoints, the request type, the body and query parameters, and the expected response format in JSON.