Update FreeBSD section of README.md

I have updated the freebsd port to include a default config.json file, which will be installed into /usr/local/etc/shadowsocks-libev by default.
To enable shadowsocks-libev on freebsd, it's as simple as adding the shadowsocks_libev_enable variable to /etc/rc.conf
Merge pull request #28 from xiaoding/patch-1

Update FreeBSD section of README.md