The recent upgrade created a broken symlink by
dropping the README.md file, as you can see here:

   $ adequate synaptic
   synaptic: broken-symlink /usr/share/doc/synaptic/README -> README.md
   $ ls -l /usr/share/doc/synaptic/README.md
   ls: cannot access '/usr/share/doc/synaptic/README.md': No such file or directory
   $ chase /usr/share/doc/synaptic/README
   chase: /usr/share/doc/synaptic/README.md: No such file or directory

This bug report brought to you by adequate:

https://bonedaddy.net/pabs3/log/2013/02/23/inadequate-software/