Add basic example for multi-compiler

examples/cli/multi-compiler/README.md

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+# CLI: Multi Compiler
+
+`webpack-dev-server` should be able to compile multiple webpack configs.
+
+```shell
+npm run webpack-dev-server -- --open
+```
+
+## What Should Happen
+
+1. The script should open `http://localhost:8080/` in your default browser.
+2. You should see the text on the page itself change to read `Success!`.
+3. In `app.js` write code containing a syntax error and save the file.
+4. The page should now refresh and show a full screen error overlay, displaying
+the syntax error.

examples/cli/multi-compiler/app.js

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+'use strict';
+
+const target = document.querySelector('#target');
+
+target.classList.add('pass');
+target.innerHTML = 'Success!';
+
+// This results in an error:
+// if(!window) require("test");

examples/cli/multi-compiler/webpack.config.js

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+'use strict';
+
+// our setup function adds behind-the-scenes bits to the config that all of our
+// examples need
+const { setup } = require('../../util');
+
+module.exports = [setup({
+  context: __dirname,
+  entry: './app.js'
+})];