Add 'Open in VS Code' badge (#102)

Author: feihugis

README.md

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 <h1 align="Center"> <p> FastSeq </p> </h1>
 
+[![Open in Visual Studio Code](https://open.vscode.dev/badges/open-in-vscode.svg)](https://open.vscode.dev/microsoft/fastseq)
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 ## Introduction
 
 FastSeq provides efficient implementation of popular sequence models (e.g. [Bart](https://arxiv.org/pdf/1910.13461.pdf), [ProphetNet](https://github.com/microsoft/ProphetNet)) for text generation, summarization, translation tasks etc. It automatically optimizes inference speed based on popular NLP toolkits (e.g. [FairSeq](https://github.com/pytorch/fairseq) and [HuggingFace-Transformers](https://github.com/huggingface/transformers)) without accuracy loss. All these can be easily done (no need to change any code/model/data if using our command line tool, or simply add one-line code `import fastseq` if using source code).