Add instructions for FreeBSD OS.

Hi,

First, thanks a lot for the Free/Libre software, it was simple compared to sysPass to install and configure!

I blogged about this for FreeBSD 12 => https://kgibran.wordpress.com/2019/10/13/teampass-installation-on-freebsd-12-with-php-7-4/

I hope these steps can be added to the official documentation, if necessary I can send a PR, do let me know.

Also, tThe official documentation needs to be updated for Debian 10, the php packages are 7.3 and some packages have been dropped in this version of PHP, like deprecation of mcrypt.