message: add text for required versions for Node.js and npm
diff: @@ -85,7 +85,7 @@ To build library yourself:
  cd surveyjs
  ```
  -2. **Acquire build dependencies.** Make sure you have [Node.js](http://nodejs.org/) installed on your workstation. This is only needed to _build_ surveyjs from sources.
  +2. **Acquire build dependencies.** Make sure you have [Node.js](http://nodejs.org/) installed on your workstation. You need a version of Node.js greater than 6.0.0 and npm greater than 2.7.0. This is only needed to _build_ surveyjs from sources.
  ```
  npm install -g karma-cli
  