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

@@ -1,5 +1,8 @@
 # Running on Hardware
 
+Prerequisite: To allow loading your unsigned binary on your personal machine,
+[secure boot] needs to be disabled in the BIOS setup.
+
 To run on real hardware you'll need a specially-prepared USB drive.
 
 ## Preparation
@@ -67,6 +70,7 @@ key.
 
 Once the one-time boot menu appears, select your USB drive and press enter.
 
+[secure boot]: https://en.wikipedia.org/wiki/UEFI#Secure_Boot
 [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