Add NPM package.json & update README

[p1-ra]

• Added npm package.json in order to be used as npm package dependency.
• Updated README, add REQUIREMENT section to specify D3.js required version

[koudelka]

Hey @p1-ra, thanks for the PR.

If you want to change the package.json to point at my repo rather than your fork, I'll merge it.

[p1-ra]

@koudelka Done. I've forget it ;)