README: document 64 bit r-pi3 variant for MACHINE

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

README

@@ -81,6 +81,7 @@ c. Set MACHINE in local.conf to one of the supported boards:
     - raspberrypi0-wifi
     - raspberrypi2
     - raspberrypi3
+    - raspberrypi3-64 (64 bit kernel & userspace)
     - raspberrypi-cm (dummy alias for raspberrypi)
     - raspberrypi-cm3 (dummy alias for raspberrypi2)
 d. bitbake rpi-hwup-image