fix(afs): fix collection add return type

docs/firestore/collections.md

@@ -312,7 +312,7 @@ For example, a user updates the third item in a list. In a state based method li
 
 ## Adding documents to a collection
 
-To add a new document to a collection with a generated id use the `add()` method. This method uses the type provided by the generic class to validate it's type structure.
+To add a new document to a collection with a generated id use the `add()` method. This method uses the type provided by the generic class to validate it's type structure.  This method returns an `AngularFirestoreDocument`, which provides methods for streaming, updating, and deleting. [See Using Documents with AngularFirestore for more information on how to use documents](documents.md).
 
 #### Basic example
 

src/firestore/collection/collection.ts

@@ -114,8 +114,8 @@ export class AngularFirestoreCollection<T=DocumentData> {
   }
 
   /**
-   * Retrieve the results of the query once. 
-   * @param options 
+   * Retrieve the results of the query once.
+   * @param options
    */
   get(options?: firestore.GetOptions) {
     return from(this.query.get(options)).pipe(
@@ -130,8 +130,8 @@ export class AngularFirestoreCollection<T=DocumentData> {
    * when you update data it is not updating data to the window of your query unless
    * the data fits the criteria of the query.
    */
-  add(data: T): Promise<DocumentReference> {
-    return this.ref.add(data);
+  add(data: T): Promise<AngularFirestoreDocument<T>> {
+    return this.ref.add(data).then(doc => new AngularFirestoreDocument<T>(doc, this.afs));
   }
 
   /**