add a little to the upgrade guide

Author: reconbot

UPGRADE_GUIDE.md

@@ -3,6 +3,7 @@ Upgrading from 4.x to 5.x
 5.x is a major rewrite to make node serialport a NodeJS stream. While the api surface is  similar there have been a number of changes to ensure more consistent error handling and operation of a serial port.
 
 - Removed the `disconnect` event. The `close` event now fires with a disconnect error object in the event of a disconnection.
+- `drain` now waits for the current javascript write to complete before calling the system level drain.
 
 The exact changes will go here see #1046