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>
diff --git a/hikey960.mk b/hikey960.mk
index 1888158..342fa04 100644
--- a/hikey960.mk
+++ b/hikey960.mk
@@ -6,6 +6,9 @@
 TARGET_PREBUILT_DTB := $(LOCAL_KERNEL_HOME)/hi3660-hikey960.dtb
 
 ifndef HIKEY_USES_GKI
+  ## Please check the following link for the android-mainline
+  ## kernel build instructions:
+  ##   https://www.96boards.org/documentation/consumer/hikey/hikey960/build/android-mainline.md.html
   ifeq ($(TARGET_KERNEL_USE), mainline)
     HIKEY_USES_GKI := true
   else