a20_olinuxino_micro/readme.txt: add link to linux-sunxi.org

Explicitly state where this information comes from.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/olimex/a20_olinuxino_lime/readme.txt

@@ -15,6 +15,8 @@ And also the mainline kernel works fine even for a basic Linux desktop
 system running on top of a simple framebuffer, which may be good enough for
 the users who do not need fancy 3D graphics or video playback acceleration.
 
+(see http://linux-sunxi.org/Linux_Kernel for more details)
+
 How to build it
 ===============