add upgrade message about verification issue

grub.install

@@ -6,11 +6,22 @@ post_upgrade() {
   if [ ! -f /boot/grub/grub.cfg -a -f /boot/grub/grub.cfg.pacsave ]; then
     mv /boot/grub/grub.cfg.pacsave /boot/grub/grub.cfg
   fi
+
+  # https://md.archlinux.org/F1JuYj5xQtWyhvH8_ilErg#
+  cat <<EOM
+:: Recent versions of grub may fail to boot with secure boot enabled. The
+   message will look like this:
+    > error: verification requested but nobody cares:
+    > (hd0,gpt2)/grub/x86_64-efi/normal.mod
+    > Entering rescue mode...
+   Handle installation to UEFI with care and be prepared!
+   For details see: https://md.archlinux.org/F1JuYj5xQtWyhvH8_ilErg#
+EOM
 }
 
 post_install() {
-  cat << 'EOM'
-Generate your bootloader configuration with:
+  cat <<EOM
+:: Generate your bootloader configuration with:
      grub-mkconfig -o /boot/grub/grub.cfg
 EOM
 }