Make a list of errata

As discussed at issue #298, I've added some remarks about versions.  The appropriate branch(es) still need to be created.  I also started some kind of errata list, with all the substantive corrections I could find that have been made since the "first-edition" tag.  For each one I listed the commit hash where it was fixed.  It would be better to list the associated number that appears in the "Book version" on the title page, so that a reader could compare it to the number in their copy and instantly know which errata they have to worry about, but I don't know how to extract that number (Andrej?).