Adding FreeBSD Port info to README.

My FreeBSD port was finally accepted.

I'm hoping to get the version bumped to latest soon and have been assured it will be accepted fairly quickly.  Not sure if you want the port pulled into the project however it can be found at:  https://github.com/rosstimson/chruby-freebsd-port

The port could also be installed with `pkg_add -r chruby` but adding both options made the instructions look a little messy.