docs: add paragraph regarding null properties in the response (#2130)

Author: edsonhoraciojunior

docs/start.md

@@ -86,6 +86,8 @@ foreach ($results as $item) {
 }
 ```
 
+**Properties are hydrated according to the value in the Response. If a property is not present in the Response it will be set to null. Some fields won't be in the Response if you didn't ask for them in the Request using the `fields` property. Therefore, watchout for null properties, maybe they have a value already but are null because they're not present in the Response.**
+
 ## Google App Engine support
 
-This library works well with Google App Engine applications. The Memcache class is automatically used for caching, and the file IO is implemented with the use of the Streams API.
+This library works well with Google App Engine applications. The Memcache class is automatically used for caching, and the file IO is implemented with the use of the Streams API.