Create screenshots and add them to readme

[tobiasdiez]

https://help.github.com/articles/relative-links-in-readmes/

• Icon after installation
• Preview window
• JabRef import

Also add screenshots to https://addons.mozilla.org/en-US/firefox/addon/jabfox/

[JaredClayborn]

I'd like to take this if you're still interested

[tobiasdiez]

Sure! That would be a great addition to get more users.