Add "Visual Studio Marketplace" badge to readme

Inspired by #533 this PR adds a "Visual Studio Marketplace" badge containing the most recent published version to the top of the readme file using [shields.io](https://shields.io/).

README.md

@@ -1,4 +1,4 @@
-# vscode-jest [![Build Status](https://travis-ci.org/jest-community/vscode-jest.svg?branch=master)](https://travis-ci.org/jest-community/vscode-jest)
+# vscode-jest [![Build Status](https://travis-ci.org/jest-community/vscode-jest.svg?branch=master)](https://travis-ci.org/jest-community/vscode-jest) [![Visual Studio Marketplace](https://img.shields.io/visual-studio-marketplace/v/Orta.vscode-jest?color=success&label=Visual%20Studio%20Marketplace)](https://marketplace.visualstudio.com/items?itemName=Orta.vscode-jest)
 
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