misc: set supported Node version to >= 6

README.md

 
 ## Building the sources
 
+Node.js >= 6 is required.
+
 On Debian/Ubuntu systems, the required packages can be installed with:
 ```
 sudo apt-get install npm nodejs-legacy

package.json

     "webpack": "2.6.1",
     "webpack-dev-server": "2.4.5"
   },
+  "engines": {
+      "node": ">=6.0.0"
+  },
   "license": "Apache-2.0",
   "scripts": {
     "lint": "jshint . && eslint . && flow",