Mention in the readme that custom HttpClient instances can be provided to HttpApiTransport

[DiscoPYF]

The readme puts emphasis on how custom transport implementations can be provided, but we should also mention that consumers can provide their own instance of HttpClient to HttpApiTransport without reimplementing the transport layer.

var client = new HttpClient(); // my own HTTP client with a bunch of custom settings
var transport = new HttpApiTransport(client);

This may actually satisfy most custom needs related to HttpClient.