Document AOT support with non-static key

[llibdude]

Just a though, feel free to ignore. But I struggled a bit how to get this library to work with AOT and a non-static key. I think it would be helpful to add an example to the README.

providers: [
    {
      provide: STRIPE_PUBLISHABLE_KEY,
      useValue: publicKeyVariable
    }
  ]

[paullryan]

Added to the documentation.

[llibdude]

thanks!