Specify node engine version requirement in package.json

Author: Joe Bottigliero

package.json

@@ -20,6 +20,9 @@
   "bugs": {
     "url": "https://github.com/webpack/webpack.js.org/issues"
   },
+  "engines": {
+    "node": ">=6.9"
+  },
   "scripts": {
     "start": "npm run init:generated && node ./bootstrap.js",
     "build": "npm run init:generated && rm -rf build/ && node ./bootstrap.js && npm run sitemap && echo webpack.js.org > build/CNAME",