Update README.md - Creating charges with an API endpoint

[zhuber]

Many people are getting hung up on the token handling portion of this package, not understanding that they need to setup an API endpoint. In my case, I wasn't sure what the API endpoint was actually supposed to do until I found some examples in the Stripe documentation. Including some quick links should cut down on the amount of "Issues" people put into the queue simply asking for help with this.

[martinratinaud]

@zhuber thanks for the update but i'm still not sure to understand how the token has to be used on the backend. Can you please provide a sample ? Thanks you