caveat: clamxav installs files into /usr/local

Specifically /usr/local/clamXav.  This actually happens after Cask
installation; the user will be prompted to install on first execution
of ClamXav, but not informed about the location.  Therefore it makes
sense to warn the user here.
Merge pull request #4302 from rolandwalker/clamxav_usr_local

caveat: clamxav installs files into /usr/local