Add snippet for Collection Group Query

* Bump `@google-cloud/firestore` to `^1.3.0` which includes the new
  functionality

Author: BenWhitehead

firestore/main/index.js

@@ -820,6 +820,65 @@ function listenErrors(db) {
   // [END listen_errors]
 }
 
+function collectionGroupQuery(db) {
+  // [START collection_group_query_data_setup]
+  var citiesRef = db.collection('cities');
+
+  var landmarks = Promise.all([
+    citiesRef.doc('SF').collection('landmarks').doc().set({
+      name: 'Golden Gate Bridge',
+      type: 'bridge'
+    }),
+    citiesRef.doc('SF').collection('landmarks').doc().set({
+      name: 'Legion of Honor',
+      type: 'museum'
+    }),
+    citiesRef.doc('LA').collection('landmarks').doc().set({
+      name: 'Griffith Park',
+      type: 'park'
+    }),
+    citiesRef.doc('LA').collection('landmarks').doc().set({
+      name: 'The Getty',
+      type: 'museum'
+    }),
+    citiesRef.doc('DC').collection('landmarks').doc().set({
+      name: 'Lincoln Memorial',
+      type: 'memorial'
+    }),
+    citiesRef.doc('DC').collection('landmarks').doc().set({
+      name: 'National Air and Space Museum',
+      type: 'museum'
+    }),
+    citiesRef.doc('TOK').collection('landmarks').doc().set({
+      name: 'Ueno Park',
+      type: 'park'
+    }),
+    citiesRef.doc('TOK').collection('landmarks').doc().set({
+      name: 'National Museum of Nature and Science',
+      type: 'museum'
+    }),
+    citiesRef.doc('BJ').collection('landmarks').doc().set({
+      name: 'Jingshan Park',
+      type: 'park'
+    }),
+    citiesRef.doc('BJ').collection('landmarks').doc().set({ 
+      name: 'Beijing Ancient Observatory',
+      type: 'museum'
+    })
+  ]);
+  // [END collection_group_query_data_setup]
+  landmarks.then((l) => console.log(l));
+
+  // [START collection_group_query]
+  let museums = db.collectionGroup('landmarks').where('type', '==', 'museum');
+  museums.get().then(function(querySnapshot) {
+    querySnapshot.forEach(function(doc) {
+      console.log(doc.id, ' => ', doc.data());
+    });
+  });
+  // [end collection_group_query]
+}
+
 // ============================================================================
 // https://firebase.google.com/docs/firestore/query-data/query-cursors
 // ============================================================================
@@ -1121,4 +1180,8 @@ describe('Firestore Smoketests', () => {
   it('should delete the whole collection', () => {
     return deleteCollection(db, 'cities', 50);
   });
+
+  it('should find all museums when querying a collection group', () => {
+    return collectionGroupQuery();
+  });
 });