Add Azure Pipelines badge to README.md

Author: Xanewok

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.com/rust-lang/rls.svg?branch=master)](https://travis-ci.com/rust-lang/rls)
+[![Build Status](https://dev.azure.com/rust-lang/rls/_apis/build/status/rust-lang.rls?branchName=master)](https://dev.azure.com/rust-lang/rls/_build/latest?definitionId=15&branchName=master)
 [![Nightly status](https://img.shields.io/badge/dynamic/json.svg?label=rls%20(Windows)&url=https%3A%2F%2Fraw.githubusercontent.com%2Frust-lang-nursery%2Frust-toolstate%2Fmaster%2F_data%2Flatest.json&query=%24%5B%3F(%40.tool%3D%3D%22rls%22)%5D.windows&colorB=lightgrey)](https://rust-lang-nursery.github.io/rust-toolstate/)
 [![Nightly status](https://img.shields.io/badge/dynamic/json.svg?label=rls%20(Linux)&url=https%3A%2F%2Fraw.githubusercontent.com%2Frust-lang-nursery%2Frust-toolstate%2Fmaster%2F_data%2Flatest.json&query=%24%5B%3F(%40.tool%3D%3D%22rls%22)%5D.linux&colorB=lightgrey)](https://rust-lang-nursery.github.io/rust-toolstate/)