docs: add demo screenshots to README

Author: gruhn

.github/screenshot1.png

@@ -10,6 +10,12 @@ A Vue.js 2 component, accessing the device camera and allowing users to read QR
 
 ### [Demos](https://gruhn.github.io/vue-qrcode-reader/)
 
+<p align="center">
+  <img src="https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/screenshot1.png" height="500" alt="Screenshot 1">
+  <img src="https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/screenshot2.png" height="500" alt="Screenshot 2">
+  <img src="https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/screenshot3.png" height="500" alt="Screenshot 3">
+</p>
+
 # Browser support
 
 | ![Internet Explorer](https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/ie_32x32.png) | ![Edge](https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/edge_32x32.png) | ![Firefox](https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/firefox_32x32.png) | ![Chrome](https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/chrome_32x32.png) | ![Safari](https://raw.githubusercontent.com/gruhn/vue-qrcode-reader/master/.github/safari_32x32.png) |