Add PRs welcome badge

Author: roberthebel

README.md

@@ -1,6 +1,7 @@
 # react-native-opencv-tutorial
 
 [![license](https://img.shields.io/badge/License-MIT-green)](https://img.shields.io/badge/License-MIT-green)
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 ## What this tutorial is about
 This tutorial is how to use React Native together with OpenCV for image processing. This example uses native Java and Objective-C bindings for OpenCV. In this example we use the device's camera to take a photo and detect whether the taken photo is clear or blurred.