Add information about adding the portability statement.

Doc/templates/module.tex

@@ -13,7 +13,7 @@
 % appropriate.
 
 \section{\module{spam} ---
-         Short descrition, for section title.}
+         Short descrition, for section title}
 
 % Choose one of these to specify the module module name.  If there's
 % an underscore in the name, use
@@ -24,6 +24,15 @@
 \declaremodule{extension}{spam}		% not standard, in C
 \declaremodule{}{spam}			% not standard, in Python
 
+% Portability statement:  Uncomment and fill in the parameter to specify the
+% availability of the module.  The parameter can be Unix, IRIX, SunOS, Mac,
+% Windows, or lots of other stuff.  When ``Mac'' is specified, the availability
+% statement will say ``Macintosh'' and the Module Index may say ``Mac''.
+% Please use a name that has already been used whenever applicable.  If this
+% is omitted, no availability statement is produced or implied.
+%
+%   \platform{UNIX}
+
 % These apply to all modules:
 
 \moduleauthor{name}{email}		% Author of the module code;