Update README.rst

README.rst

@@ -77,6 +77,10 @@ You can set any duration as the shelf life of cached return values of a function
   @cachier(stale_after=datetime.timedelta(weeks=2))
   def bar(arg1, arg2):
     return {'arg1': arg1, 'arg2': arg2}
+    
+Now when a cached value matching the given arguments is found the time of its calculation is checked; if more than ``stale_after`` time has since passed the function will be run again for the same arguments and the new value will be cached and returned.
+
+This is usefull for lengthy calculation that depend on a dynamic data source.
 
 Fuzzy Shelf Live
 ~~~~~~~~~~~~~~~~