board: raspberrypi: mention the Zero model in readme.txt

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/raspberrypi/readme.txt

@@ -22,6 +22,10 @@ For models A, B, A+ or B+:
 
   $ make raspberrypi_defconfig
 
+For model Zero (model A+ in smaller form factor):
+
+  $ make raspberrypi0_defconfig
+
 For model 2 B:
 
   $ make raspberrypi2_defconfig