The answer depends on what specifically you are wanting to provide.

If you are just looking for the main part of the readme which lists the modified and new signatures, then you can download the latest one and it has all that information for the last several sig updates:

Here is the link to the S407 Readme

http://www.cisco.com/web/software/282549755/27019/IPS-sig-S407.readme.txt

You can look to the bottom and find the Sig information all the way back to S339.

If you are looking for a quick way for your coworkers to see the signature list for upcoming sig updates, then check the Cisco IPS Active Update Bulletins Archive on cisco.com:

http://tools.cisco.com/security/center/bulletin.x?i=57

Each Bulletin will list the signature modified or new in the signature update.

They are date labeled instead of sig update release labeled.

If you want the actual readme files for the signature updates, then you might also try going to this page:

http://www.cisco.com/cgi-bin/tablebuild.pl/ipsmc-ips5-sigup

This is the page where signature update files can be manually downloaded for the VMS or CSM management tools.

The signature readme files posted here are also the same for the sensor.

The advantage that this page has, is that all of the files can at least be accesssed from a single page.

NOTE: Older readmes can be accessed in the archive location for the above page:

http://www.cisco.com/cgi-bin/tablebuild.pl/ipsmc-ips-sigup-arch

Hope one of these options will work for you.