Update readme

README.md

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 # qr_image
+
+A website to generate valid QR codes with images in the background.
+
+### Usage
+
+1. Open [site](https://hughchen.github.io/qr_image/).
+2. Upload an image.
+3. Enter the URL.
+4. Adjust parameters.
+5. Download `PNG`.
+
+## Dependency
+
+* [davidshimjs / qrcode](https://github.com/davidshimjs/qrcodejs)