Add getting started section to README.md

[Ryuno-Ki]

This is a follow-up of #8.

It took me a short moment to learn, how to get started with this project.
In the end

Github-organization-inviter/main.js

Line 1 in 5381d04

#!/usr/bin/env nodejs

gave me the final hint, that I had to call node ./main.js to start the express server.

Normally I'd expect to find a npm start in package.json (#15) to run the server.

[ionicc]

Yes, This goes with #15. The addition of package.json would fix this.