improve readability of README

[RvanNieuwpoort]

The readme immediately goes into a lot of detail, without a high-level description of what the purpose of this sofware is. There currently is only one line describing the purpose:

"Python package to analyze a GitHub or GitLab repository's compliance with the fair-software.eu recommendations."

Maybe that is clear for insiders, but for the target audience this may be to short. A bit of context on the fair principles for software, why those are useful, and how this tool helps users to implement them would be great.