title: How to use startAfter on Firestore?
body: <p>I want to update my gridview and get data from firestore with startAfter. 
  My code how I get first data is below. 
  How to use startAfter in next query?</p>
  
  <pre><code>Firestore.instance.collection('Posts').limit(6).getDocuments().then((snap){
        postList.clear();
        for(int i=0; i&lt;snap.documents.length; i++){
          Post post = new Post(snap.documents[i]['imageUrl'],
              snap.documents[i]['userId'],
              snap.documents[i]['color'],
              snap.documents[i]['brand'],
              snap.documents[i]['sex'],
              snap.documents[i]['category']);
          postList.add(post);
        }
        setState(() {
          print('Length : $postList.Lenth');
        });
      });
  </code></pre>
  
  <p>Next query.</p>
  
  <pre><code>Firestore.instance.collection('Posts').startAfter(whats here?).limit(8).getDocuments().then((snap){
          for(int i=0; i&lt;snap.documents.length; i++){
            Post post = new Post(snap.documents[i]['imageUrl'],
                snap.documents[i]['userId'],
                snap.documents[i]['color'],
                snap.documents[i]['brand'],
                snap.documents[i]['sex'],
                snap.documents[i]['category']);
            postList.add(post);
          }
          setState(() {
            print('Length : $postList.Lenth');
          });
        });```
  </code></pre>