Add "XS" to glossary

S99-glossary.pod

@@ -2793,6 +2793,13 @@ channel that was intended for another channel, or for a private message.
 
 =head1 X
 
+=head2 XS
+
+Perl 5's mechanism for bridging the gap between pure-Perl code and compiled
+system libraries. Writing XS modules involves C code and some knowledge of Perl
+5's inner workings, and installing them needs a compiler. The Perl 6 equivalent
+is C<NativeCall>; see L<S21|S21: Calling foreign code>.
+
 =head2 XY Problem
 
 Sometimes people want to achieve X, and think Y is the best way to do it. So