Use CI badge from img.shields.io

Work around microsoft/vscode-vsce#389.
Currently, we can't use SVG shields directly from github.com, so
let's temporarily use whitelisted img.shields.io.

Author: Xanewok

README.md

@@ -1,7 +1,7 @@
 # Rust support for Visual Studio Code
 
 [![](https://vsmarketplacebadge.apphb.com/version/rust-lang.rust.svg)](https://marketplace.visualstudio.com/items?itemName=rust-lang.rust)
-![VSCode + Node.js CI](https://github.com/rust-lang/rls-vscode/workflows/VSCode%20+%20Node.js%20CI/badge.svg)
+[![VSCode + Node.js CI](https://img.shields.io/github/workflow/status/rust-lang/rls-vscode/VSCode%20+%20Node.js%20CI.svg?logo=github)](https://github.com/rust-lang/rls-vscode/actions?query=workflow%3A%22VSCode+%2B+Node.js+CI%22)
 
 Adds language support for Rust to Visual Studio Code. Supports: