Update README.md file

[muyunli123]

As a user of the recommendation API I need instructions on functionalities of the API So that I know what calls are available and what inputs they expect and how to run and test my code

Details and Assumptions

• overview and objectives of the recommendation API

• demo on API calls

  Acceptance Criteria

  Given the current state of the README.md file
  When the file is updated with the necessary information
  Then the file should contain objectives of the recommendation API and clear instructions on API calls