Add some material to README-1ST, along with a couple of formatting changes.
Add some material to README-1ST, along with a couple of formatting changes.

Give an early pointer to where to look if 'acprep update' fails.
Clarify a few things in the first section.
Give sections clearer headers, so they can be more easily referred to.
Add a FAQ entry about what to do if './acprep update' gives errors.
Update FAQ entry about MPFR library to refer to above new entry.

afh/ledger,ledger/ledger,ledger/ledger,ledger/ledger,paulbdavis/ledger,paulbdavis/ledger,duncanmortimer/ledger,jwakely/ledger,paulbdavis/ledger,jwakely/ledger,jwakely/ledger,ledger/ledger,duncanmortimer/ledger,duncanmortimer/ledger,jwakely/ledger,duncanmortimer/ledger,duncanmortimer/ledger,afh/ledger,jwakely/ledger,paulbdavis/ledger,paulbdavis/ledger,paulbdavis/ledger,paulbdavis/ledger,afh/ledger,duncanmortimer/ledger,duncanmortimer/ledger,jwakely/ledger,afh/ledger,afh/ledger,ledger/ledger,jwakely/ledger