add pr-welcome badge :+1:

README.md

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 [![Join the chat at https://gitter.im/Microsoft/ChakraCore](https://badges.gitter.im/Microsoft/ChakraCore.svg)](https://gitter.im/Microsoft/ChakraCore?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Licensed under the MIT License](https://img.shields.io/badge/License-MIT-blue.svg)](https://github.com/Microsoft/ChakraCore/blob/master/LICENSE.txt)
+[![PR's Welcome](https://img.shields.io/badge/PRs%20-welcome-brightgreen.svg)](https://github.com/Microsoft/ChakraCore/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-desc)
 
 ChakraCore is the core part of Chakra, the high-performance JavaScript engine that powers Microsoft Edge and Windows applications written in HTML/CSS/JS.  ChakraCore supports Just-in-time (JIT) compilation of JavaScript for x86/x64/ARM, garbage collection, and a wide range of the latest JavaScript features.  ChakraCore also supports the [JavaScript Runtime (JSRT) APIs](https://github.com/Microsoft/ChakraCore/wiki/JavaScript-Runtime-%28JSRT%29-Overview), which allows you to easily embed ChakraCore in your applications.