docs/esp8266: Add ntptime usage to esp8266 quickref.

docs/esp8266/quickref.rst

@@ -243,6 +243,12 @@ See :ref:`machine.RTC <machine.RTC>` ::
     rtc.datetime((2017, 8, 23, 1, 12, 48, 0, 0)) # set a specific date and time
     rtc.datetime() # get date and time
 
+    # synchronize with ntp
+    # need to be connected to wifi
+    import ntptime
+    ntptime.settime() # set the rtc datetime from the remote server
+    rtc.datetime()    # get the date and time in UTC
+
 Deep-sleep mode
 ---------------