Document minimally supported PHP version for ISPConfig itself

We list "Supported operating systems" in README.md.

But the minimal PHP version we want to support with our code is not easy to find. Can we add that info in the README?

Debian 9 and Ubuntu 16.04 came with php 7.0, for centos 7 it was 5.4 for what I could find.

So I guess it's 5.4 currently ... support for centos 7 ended last year, so maybe we can re-evaluate that...