introduction to nonnormative section added

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/packages.tex

@@ -352,6 +352,9 @@ \section{Multilingual Descriptions}\label{multilingual-descriptions}
 The detailed format of these files is described in \href{https://www.gnu.org/software/gettext/manual/gettext.pdf}{GNU gettext}.
 
 \begin{nonnormative}
+In the remainder of this section, several facts about the gettext specification are described non-normatively for easy access to some of the gettext basics.
+Always refer to the external gettext specification for full detail or in case of doubt.
+
 All text strings are in double quotes and encoded with UTF-8 characters.
 Comments start with an \lstinline!#! and are continued until the end of line.
 Spaces outside strings are ignored and used as separators.