Added documentation for AuthenticatedParameterizedHttpClientHandler w…

…here signature generation is dependent on contents of the HTTP request

README.md

@@ -418,7 +418,9 @@ var user = await GetUser("octocat", "token OAUTH-TOKEN");
 #### Authorization (Dynamic Headers redux)
 The most common reason to use headers is for authorization. Today most API's use some flavor of oAuth with access tokens that expire and refresh tokens that are longer lived.
 
-One way to encapsulate these kinds of token usage, a custom `HttpClientHandler` can be inserted instead. 
+One way to encapsulate these kinds of token usage, a custom `HttpClientHandler` can be inserted instead.  
+There are two classes for doing this: one is `AuthenticatedHttpClientHandler`, which takes a `Func<Task<string>>` parameter, where a signature can be generated without knowing about the request.
+The other is `AuthenticatedParameterizedHttpClientHandler`, which takes a `Func<HttpRequestMessage, Task<string>>` parameter, where the signature requires information about the request (see earlier notes about Twitter's API)
 
 For example:
 ```csharp
@@ -447,6 +449,34 @@ class AuthenticatedHttpClientHandler : HttpClientHandler
 }
 ```
 
+Or:
+
+```csharp
+class AuthenticatedParameterizedHttpClientHandler : DelegatingHandler
+    {
+        readonly Func<HttpRequestMessage, Task<string>> getToken;
+
+        public AuthenticatedParameterizedHttpClientHandler(Func<HttpRequestMessage, Task<string>> getToken, HttpMessageHandler innerHandler = null)
+            : base(innerHandler ?? new HttpClientHandler())
+        {
+            this.getToken = getToken ?? throw new ArgumentNullException(nameof(getToken));
+        }
+
+        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            // See if the request has an authorize header
+            var auth = request.Headers.Authorization;
+            if (auth != null)
+            {
+                var token = await getToken(request).ConfigureAwait(false);
+                request.Headers.Authorization = new AuthenticationHeaderValue(auth.Scheme, token);
+            }
+
+            return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
+        }
+    }
+```
+
 While HttpClient contains a nearly identical method signature, it is used differently. HttpClient.SendAsync is not called by Refit. The HttpClientHandler must be modified instead.
 
 This class is used like so (example uses the [ADAL](http://msdn.microsoft.com/en-us/library/azure/jj573266.aspx) library to manage auto-token refresh but the principal holds for Xamarin.Auth or any other library: