Streamline and maybe move installation instructions

[aaaaalbert]

The wizard's landing page asks to pip install in-toto from GitHub. It should probably suggest to do so in a virtualenv to keep it nicely isolated from the rest of the probably-fragile local Python install.

[aaaaalbert]

To clarify, IMHO most projects now default to including virtualenv in their installation instructions, and let people that don't want to use it figure it out.

[lukpueh]

More installation clarification is needed, e.g. about supported Python version - #14 (comment), supported operating systems and system requirements.

[lukpueh]

There was the suggestion to postpone the installation instruction until the user actually needs in-toto, so as to not discourage the user with such a hurdle in the beginning.

I think that is a good idea, but should at least mention in the very beginning that the wizard will require the user to install in-toto, so that it does not come as a surprise.