State installation prerequesites in docs #140

In #139 @christiansiegel wrote:

Some things we might look into before releasing on PyPI:

• support and tests for more than one python version (currently only 3.8 is used in the docker build/test; I think even 3.7 doesn't work at the moment)
• support windows (currently the temporary directory for git checkout is hardcoded to /tmp)
• add proper error message if there is no git installed (currently probably results in an ugly exception)

Currently the Setup Page does not indicate any specific prerequisites for installing from source - this should be changed for usability.