chore(ci): Add a status badge for the azure pipelines CI build (#601)

This proposes a status badge in the README.md for the Azure Pipelines CI build.

README.md

@@ -7,6 +7,7 @@
 
 [![npm](https://img.shields.io/npm/v/webpack-cli.svg)](https://www.npmjs.com/package/webpack-cli)
 [![Build Status](https://travis-ci.org/webpack/webpack-cli.svg)](https://travis-ci.org/webpack/webpack-cli)
+[![Build2 Status](https://dev.azure.com/webpack/webpack/_apis/build/status/webpack.webpack-cli)](https://dev.azure.com/webpack/webpack/_build/latest?definitionId=4)
 [![Dependency Status](https://david-dm.org/webpack/webpack-cli.svg)](https://david-dm.org/webpack/webpack-cli)
 [![Code Climate](https://codeclimate.com/github/webpack/webpack-cli/badges/gpa.svg)](https://codeclimate.com/github/webpack/webpack-cli)
 [![chat on gitter](https://badges.gitter.im/webpack/webpack.svg)](https://gitter.im/webpack/webpack)