hikey960: add link for mainline kernel build instructions

Since the android-mainline branch is not officially supported
in AOSP, but Linaro does some work there, so share the Linaro
instructions here for people interested in to refer to

Test: booted to homescreen

Change-Id: I8746ecec7882a0e1eacd7cacf25de1cca8c2d49d
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
1 file changed