asciidoc: Add caveat about XML catalog

A standard installation of asciidoc will cause manpage generation using a2x to fail, because xmllint cannot find the XML catalog installed by asciidoc.

See: http://grepalex.com/2013/02/17/installing-asciidoc-on-osx/

This commit adds a caveat to the asciidoc brew, which will hopefully save the next user all the Googling I just had to do...