http://vietunicode.sourceforge.net/charset/quytacABC_en.html
is documentation from the Vietnam Lexicography Centre about Vietnamese
sorting rules for dictionary entries.

Thanks to Pavel Striz <pavel at striz.cz> for supplying that
information at 2014-03-28 in the xindy mailing list and also sending a
patch that fixes the former incorrect sorting.

Comment from Joachim Schrod <jschrod@acm.org> at 2014-04-19: I have
not checked if rule 4 and Note 1 from that documentation are really
realized by xindy. If somebody could check and tell me, I would be
grateful.
