Log when refresh of playlists starts (mopidy#311)

I have ~1k playlists on my account so a refresh takes about 8 minutes.
The logs gives no indication what is happening and I was confused so
providing an explanation to what is happening would have been useful.

Author: simeg

mopidy_spotify/playlists.py

@@ -53,6 +53,8 @@ def refresh(self):
         if not self._backend._web_client.logged_in:
             return
 
+        logger.info("Refreshing Spotify playlists")
+
         with utils.time_logger("playlists.refresh()", logging.DEBUG):
             _sp_links.clear()
             self._backend._web_client.clear_cache()