Add screenshots to README

[bwitt]

It'd be nice to have screenshots in the README to show what things look like. Maybe:

• main page
• group page
• node page
• report listing for a node
• report view

[gdubicki]

+💯 and let me explain why:

I am a user of voxpupuli/puppetboard and perhaps I would switch to your app but it's hard to make this decision without the screenshots that would show the features. I don't want to invest my time into trying to set it up without knowing what I will get...