Add API documentation

[shubhbapna]

Describe the solution you'd like
Document each API (method, path, any path params, any queries, any request body, responses etc)

[ggggg]

In README or in code comments?

[huonggiangbui]

I think we might want another doc (another .md)!

[ggggg]

Alrighty, can you please assign me to this issue?