Provide README and OpenAPI specification

Summary:
The OpenAPI specification follows Swagger 2.0, as there is currently
more tools (documentation, code generators) compatible with this version.

Test Plan:
Documentation generation has been tested with Spectacle.

Client code generation has been tested with swagger-codegen,
using the bash flavour.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1881