Add a note to Doc/docbook-xml42 requirement in docbook/README.txt indicating
that a later version may be required; e.g. my a2x (from cygwin64) generates a
developer-guide.xml that needs docbook-xml45.

Change-Id: Iea9d4b45e32157a9317e3fcf6794668e3a810c54
Reviewed-on: https://code.wireshark.org/review/2750
Reviewed-by: Evan Huus <eapache@gmail.com>