Proposal: new readme

This is a proposal for a slightly polished and updated Readme. I took out the quick install guide, it should be moved to https://wiki.diasporafoundation.org/Installation/Script IMO, which is linked from https://wiki.diasporafoundation.org/Installation

As I'm not a native speaker review from that side is much appreciated.

Any further suggestions what would be must haves for the readme? Do you dislike anything I did include?