title: Firestore query for string that contains the given keyword swift
body: <p>I am trying to query the firestore database to find documents with attribute string that "contains" the given keyword.</p>
  
  <p>For instance, if I have the keyword an, I want documents with attribute value such as "and", "ande", "any" to show up.</p>
  
  <p>However, the Firestore doc only states that there's only equalTo and greater and less than operations.</p>
  
  <p>How can I accomplish this feat?</p>