Add comment explaining why we wait before reading

Author: SamantazFox

adafruit_mcp2515/__init__.py

@@ -406,6 +406,10 @@ def unread_message_count(self):
         Returns:
             int: The unread message count
         """
+
+        # Wait until the queue is empty before reading from the MCP2515 again,
+        # otherwise we'll fill it faster than what the user app can process
+        # and quickly run out of memory.
         if len(self._unread_message_queue) == 0:
             self._read_from_rx_buffers()