hdis86-0.1: doc/customizing.txt

If you've built udis86 with a custom x86optable.xml, or you're using a version
other than 1.7, you may need to regenerate Hdis86/Internal/Opcode.hs to keep
it synchronized.

To do this, run the script tools/gen_opcode.sh, passing it the path to
libudis86/itab.h as a command-line argument.  It will produce a suitable
Opcode.hs on standard output.