Add warning when HOST environment variable is set (facebook#3719)

packages/react-scripts/scripts/start.js

@@ -51,6 +51,19 @@ if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
 const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
 const HOST = process.env.HOST || '0.0.0.0';
 
+if (process.env.HOST) {
+  console.log(
+    chalk.yellow(
+      "Attempting to bind to $HOST because it was specified. If this was unintentional, check that you haven't mistakenly set it in your shell."
+    )
+  );
+  console.log(
+    chalk.cyan(
+      'https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#advanced-configuration\n'
+    )
+  );
+}
+
 // We attempt to use the default port but if it is busy, we offer the user to
 // run on a different port. `choosePort()` Promise resolves to the next free port.
 choosePort(HOST, DEFAULT_PORT)