title: Firestore - Rest API:: Add new field in existing document
body: <p>Is it possible to add a new field to an existing document via restful API?</p>
  <p>For example:</p>
  <p>Existing document:</p>
  <p>XYZ { color : blue,
  age: 25 }</p>
  <p>Make a restful API to turn it into:</p>
  <p>XYZ { color : blue,
  age: 25,
  city: LA }</p>
  <p>Without having to upload all the existing data again?</p>