Update documentation around custom authentication

Description

According to the Authentication Documentation I should be able to create my own credential object and pass that into whatever client I need. Unfortunately this doesn’t appear to be supported or at least it’s a round about way that isn’t documented for this library.

Expected

// Create the Google Credentials
var credential = GoogleCredential.FromAccessToken(Secrets.GoogleAccessToken);

// Create the Client with a specified credential
var client = ImageAnnotatorClient.Create(credential);

// Get authenticated response
var response = client.DetectLabels(image); 

Actual

Credentials must be a service account and the json file must be exported to as an environment variable. Which means that you cannot use an Access Token and this couldn’t be a resource that I locate in a custom manner.

[jskeet]

Use new PublisherServiceApiClientBuilder { ... }.Build().

[jengstro2]

In a relation to use with PublisherServiceApiClient.Create() ?