[SYSTEMML-1051] Update README.md with Node.js Requirements
[SYSTEMML-1051] Update README.md with Node.js Requirements

Update the README.md mentioning that latest version of Node.js
is required. Also update commands to properly install dependencies
using the global tag which requires sudo.

gweidner/systemml-website,gweidner/systemml-website,apache/incubator-systemml-website,apache/incubator-systemml-website,objectadjective/incubator-systemml-website,objectadjective/incubator-systemml-website