In this directory, each individual package should install it's
menu files. Although changing them in this directory to change
the menus on your local system may work, the sysadmin (you?)
is best advised to put the local menufiles in /etc/menu, as, 
otherwise your changes are deleted when you upgrade the corresponding
packages. Users may override the system-wide defaults by putting
their menuentry files in ~/.menu.


For the package maintainer that wants to include menu files in his
package:

The files should have the name of the package that's installing it,
and may contain as may lines (menu entries) as is necessary.

For examples, please look in /usr/lib/menu/default

Also, note that the files in this directory are probably mostly in
the "old" format, and if you are writing new menu entries, please use
new-format menu entries like those found in /usr/lib/menu/default, i.e.
ones that look like:

?package($p):command=/bin/ls section="Apps/System" needs=text title=ls

Also, in your postinst you should check for the availability of
the update-menus command, and if available, execute it (no
parameters needed).

For more info, please read /usr/doc/menu/html