Improve README

[raphiz]

• Add a screenshot
• Installation / pypi link
• More documentation on address source (inline!)
• Update "Make a New Release" section?

[fabianhauser]

• Installation / pypi link
• More documentation on address source (inline!)
• Update "Make a New Release" section?

[Nino678190]

I think you could change the vim part into any terminal Texteditor with examples like vim, nano, etc.