Update docs stating the need to call cleanup (#550)

Author: Maragues

database/src/main/java/com/firebase/ui/database/FirebaseRecyclerAdapter.java

@@ -67,6 +67,9 @@
  *     };
  *     recycler.setAdapter(mAdapter);
  * </pre>
+ * <p>
+ * To avoid Context leaks, make sure you invoke {@link #cleanup() cleanup}
+ * <p>
  *
  * @param <T>  The Java class that maps to the type of objects stored in the Firebase location.
  * @param <VH> The ViewHolder class that contains the Views in the layout that is shown for each object.