Add installation instructions for FreeBSD [ci skip]

Hello,

As @fxn request [here](https://github.com/rails/docrails/commit/1565821c3432b5723909bf8b3b08d4230efa42f6#commitcomment-3732864), I took the liberty to add instructions for FreeBSD users. I have commit rights on docrails but open this pull request to make sure everything is right. Here are some notes:

* The examples are using the root prompt since the sudo command [should be installed](http://www.freshports.org/security/sudo/). If you want me to update the examples with `sudo`, let me know.
* I haven't put the commands for ports ; only for packages. However, there is the port's name in each case but I can update it if you want.
* Finally, the PostgreSQL version is 9.2 in the patch but the `postgresql93-{client,server}` packages are available but it's [still a beta](http://www.postgresql.org/about/news/1471/).

Have a nice day.