title: What is the JSON format for a firestore composite index including an array?
body: <p>I want to filter a collection (let's call it <code>documents</code>) using <code>array-contains</code> on one column (say <code>keywords</code>) and sort by another column (say <code>name</code>). </p>
  
  <p>I am able to create this composite index in the firebase console, but I can only guess at the format for adding it to <code>firestore.indexes.json</code>.</p>
  
  <p>It's unfortunate we can't download the index file from the console.</p>