Add 30 min flag to export_labels. (#22)

Co-authored-by: Alexandra Cota <alexandracota@Alexandras-MBP.localdomain>

Author: alexandracota

labelbox/schema/project.py

@@ -112,6 +112,8 @@ def export_labels(self, timeout_seconds=60):
         """ Calls the server-side Label exporting that generates a JSON
         payload, and returns the URL to that payload.
 
+        Will only generate a new URL at a max frequency of 30 min.
+        
         Args:
             timeout_seconds (float): Max waiting time, in seconds.
         Returns: