Update CONTRIBUTING.md and fix dependencies

[puresick]

Working on Issue 406, I noticed a few pieces of information missing to properly setup the project to be able to run it locally.

This PR updates the following things:

• Missing information about the development setup has been added to CONTRIBUTING.md which are needed to setup the project and get it running
  • To determine the latest Node.js version the current used vuepress version is working with, I tested all the releases listed on the Node.js project page
  • Added .node-version configuration file for Node.js version managers to ensure correct version is used
• Missing project dependency SockJS-client has been added to package.json and package-lock.json