docs/esp8266/quickref: Add quickref info for RTC class.

docs/esp8266/quickref.rst

@@ -223,6 +223,17 @@ and is accessed via the :ref:`machine.I2C <machine.I2C>` class::
     buf = bytearray(10)     # create a buffer with 10 bytes
     i2c.writeto(0x3a, buf)  # write the given buffer to the slave
 
+Real time clock (RTC)
+---------------------
+
+See :ref:`machine.RTC <machine.RTC>` ::
+
+    from machine import RTC
+
+    rtc = RTC()
+    rtc.datetime((2017, 8, 23, 1, 12, 48, 0, 0)) # set a specific date and time
+    rtc.datetime() # get date and time
+
 Deep-sleep mode
 ---------------