Mention to turn off secure boot.

On my Thinkpad X260, it just refused to start. No error message was displayed. It was absolutely unclear whether my binary crashed or there was another problem. The problem was indeed secure boot. Since no error message was displayed, I think it's nice to mention this pitfall here to avoid some frustration.

Author: diekmann

book/src/tutorial/hardware.md

@@ -67,6 +67,10 @@ key.
 
 Once the one-time boot menu appears, select your USB drive and press enter.
 
+If this personally-built, unsigned binary fails to load,
+[secure boot] needs to be disabled.
+
 [GPT]: https://en.wikipedia.org/wiki/GUID_Partition_Table
 [FAT]: https://en.wikipedia.org/wiki/File_Allocation_Table
 [boot files]: ../concepts/gpt.html#system-partition
+[secure boot]: https://en.wikipedia.org/wiki/UEFI#Secure_Boot