Add screenshots to readme

[bitjson]

We should add some screenshots of the QR scanner in use with various UIs. Might help to get the message across that the UI is implemented in browserland.

[bitjson]

If anyone is willing to contribute a screenshot of cordova-plugin-qrscanner working in your app, please drop it in this issue!

[bitjson]

Just added screenshots for both of BitPay's projects: 14ed4dc

Could anyone else provide screenshots of the scanning interfaces you've developed? If you wouldn't mind uploading a screenshot to this GitHub issue (and providing a link if your project is public), I can get it included in the readme.

(Maybe @tobiasviehweger, @zimzimple, @stefanhuber, @elledienne, or @majochoc?)

Thanks!