docs: gxbb: Add documentation

Change-Id: Ie2465c1ccc482bd8eb5e5a71c580543095e4ba94
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/readme.rst b/readme.rst
index f2a7a00..e7fbfb4 100644
--- a/readme.rst
+++ b/readme.rst
@@ -193,6 +193,7 @@
 This release also contains the following platform support:
 
 -  Allwinner sun50i_64 and sun50i_h6
+-  Amlogic Meson S905 (GXBB)
 -  ARM SGI-575 and SGM-775
 -  HiKey, HiKey960 and Poplar boards
 -  Marvell Armada 8K