Undocumented context needed to run some utility scripts

[hwine]

Some scripts committed to the repository are actually required to support autograph operations at Mozilla (albeit on a very infrequent basis). These scripts are not able to be run "as is", as they require undocumented prerequisites.

For example, autograph/tools/autograph-client/build_test_xpis.sh is one such script that produces signed artifacts that need to be updated when the root or intermediate certificates are changed.