Add screenshot of plugin in action in readme

[matkoniecz]

It not only makes readme more attractive (at least to me) but also shows what should be expected after installation.

I had some minor installation problem (I needed to modify settings for it to work) and one of things that I investigated was that by default it is subtle or hides output somewhere. With screenshoot in readme I would be able to discard this possibility immediately.

I may make PR fixing this issue.

[pderichs]

Had the same idea when I started working on the plugin, but didn't took the time to record something until now, so I like the idea 👍