Add a new Handbook chapter about FreeBSD jails. It has been originally written
by Matteo Riondato and further work has been done by me and my great mentors.
Despite it took months to get it in, we believe it's a right time to commit and
have a wider audience to look at it, and improve it even more. I hope you will
like it :-)

Approbed by: trhodes (mentor), keramida (mentor)