Add twitter badge to README.md
<p align="center"><img width="70%" src="docs/source/\_static/img/parlai.png" /></p>
-[![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/facebookresearch/ParlAI/blob/master/LICENSE) [![CircleCI](https://circleci.com/gh/facebookresearch/ParlAI.svg?style=shield)](https://circleci.com/gh/facebookresearch/ParlAI/tree/master) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://github.com/facebookresearch/ParlAI/blob/master/CONTRIBUTING.md)
+[![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/facebookresearch/ParlAI/blob/master/LICENSE) [![CircleCI](https://circleci.com/gh/facebookresearch/ParlAI.svg?style=shield)](https://circleci.com/gh/facebookresearch/ParlAI/tree/master) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://github.com/facebookresearch/ParlAI/blob/master/CONTRIBUTING.md) [![Twitter](https://img.shields.io/twitter/follow/parlai_parley?label=Twitter&style=social)](https://twitter.com/parlai_parley)
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