document installation in the readme

[travi]

the steps are simple

• nvm install
• npm install

but they arent in the readme, so they might not be obvious to all contributors

link to the org level contributing file for setup of things like nvm and git cz

[travi]

the info has been added to the readme, but we should still link to the org level docs once https://github.com/dsmjs/meta/issues/3 is taken care of