fix: Update readme with details on supply a custom HttpClient instance (

#1362)

Co-authored-by: Chris Howarth <christopher.howarth@autocab.com>
Co-authored-by: Glenn <5834289+glennawatson@users.noreply.github.com>

README.md

@@ -67,6 +67,7 @@ services
   * [Headers inheritance](#headers-inheritance)
 * [Default Interface Methods](#default-interface-methods)
 * [Using HttpClientFactory](#using-httpclientfactory)
+* [Providing a custom HttpClient](#providing-a-custom-httpclient)
 * [Handling exceptions](#handling-exceptions)
   * [When returning Task<IApiResponse>, Task<IApiResponse<T>>, or Task<ApiResponse<T>>](#when-returning-taskiapiresponse-taskiapiresponset-or-taskapiresponset)
   * [When returning Task<T>](#when-returning-taskt)
@@ -1155,6 +1156,48 @@ public class HomeController : Controller
 }
 ```
 
+### Providing a custom HttpClient
+
+You can supply a custom `HttpClient` instance by simply passing it as a parameter to the `RestService.For<T>` method:
+
+```csharp
+RestService.For<ISomeApi>(new HttpClient()
+{
+    BaseAddress = new Uri("https://www.someapi.com/api/")
+});
+```
+
+However, when supplying a custom `HttpClient` instance the following `RefitSettings` properties will not work:
+
+* `AuthorizationHeaderValueGetter`
+* `AuthorizationHeaderValueWithParamGetter`
+* `HttpMessageHandlerFactory`
+
+If you still want to be able to configure the `HtttpClient` instance that `Refit` provides while still making use of the above settings, simply expose the `HttpClient` on the API interface:
+
+```csharp
+interface ISomeApi
+{
+    // This will automagically be populated by Refit if the property exists
+    HttpClient Client { get; }
+
+    [Headers("Authorization: Bearer")]
+    [Get("/endpoint")]
+    Task<string> SomeApiEndpoint();
+}
+```
+
+Then, after creating the REST service, you can set any `HttpClient` property you want, e.g. `Timeout`:
+
+```csharp
+SomeApi = RestService.For<ISomeApi>("https://www.someapi.com/api/", new RefitSettings()
+{
+    AuthorizationHeaderValueGetter = () => GetTokenAsync()
+});
+
+SomeApi.Client.Timeout = timeout;
+```
+
 ### Handling exceptions
 Refit has different exception handling behavior depending on if your Refit interface methods return `Task<T>` or if they return `Task<IApiResponse>`, `Task<IApiResponse<T>>`, or `Task<ApiResponse<T>>`.