Update FreeBSD installation instructions

the_silver_searcher is now in [ports](http://www.freshports.org/textproc/the_silver_searcher/). This commit adds instructions to install from binary packages as well as updates the source installation instructions to use ports.

Installation from source instructions are moved under the "Installation" heading as building Ag from FreeBSD ports is supported by FreeBSD in the same sense that building Ag from portage is supported by Gentoo.