Update README with ngrok instructions

After getting to the last part where http://localhost:8081/prestashop is up and running. Instructions could be added to use https://ngrok.com/ and allow external use via commands like: `./ngrok -subdomain psMyFirstName 8081` that allow anyone to reach your server at http://psMyFirstName.ngrok.com/prestashop

This will blow away your co-workers with its ease of use!