Update README.md for farmdata2_api

[JingyuMarcelLee]

The README.md file in farmdata2/farmdata2_api needs to be updated so that it covers the Express API addition to FarmData2.

Below are the tentative topics that need to be covered

• Explain that the Swagger documentation for the APIs available at fd2_api/docs
• Explain how to create API endpoints
  • Automatic generation of Swagger documentation must follow OpenAPI Specifications. (Links to this should be included)
  • Conventions for endpoint naming and paths
  • How to write SQL Queries and how to test them using phpMyAdmin
• How to test the endpoints (Testing is not available yet. TODO)
• How to refactor FarmOSAPI.js to update the mapping functions used in pages