Show warning console if version.js file is missing (#678)

lib/version.js

@@ -33,6 +33,16 @@ program
   })
   .parse(process.argv);
 
+const hasVersionFile = fs.existsSync(CWD + '/pages/en/versions.js');
+if (!hasVersionFile) {
+  console.error(
+    `${chalk.yellow(
+      'No versions.js file found!'
+    )}\nYou should create your versions.js file in pages/en directory.\nPlease refer to https://docusaurus.io/docs/en/versioning.html.`
+  );
+  process.exit(1);
+}
+
 if (typeof version === 'undefined') {
   console.error(
     `${chalk.yellow(