add examples for read using custom decoder per type, read using custom decoder via iris#UnmarshalerFunc and to complete it add an example for the context#ReadXML.

Former-commit-id: 536b1780f12d0b9d9ce9aa976a0f95f18634ec2d

go19.go

@@ -17,6 +17,16 @@ type (
 	// Developers send responses to the client's request through a Context.
 	// Developers get request information from the client's request by a Context.
 	Context = context.Context
+	// UnmarshalerFunc a shortcut, an alias for the `context#UnmarshalerFunc` type
+	// which implements the `context#Unmarshaler` interface for reading request's body
+	// via custom decoders, most of them already implement the `context#UnmarshalerFunc`
+	// like the json.Unmarshal, xml.Unmarshal, yaml.Unmarshal and every library which
+	// follows the best practises and is aligned with the Go standards.
+	//
+	// See 'context#UnmarshalBody` for more.
+	//
+	// Example: https://github.com/kataras/iris/blob/master/_examples/http_request/read-custom-via-unmarshaler/main.go
+	UnmarshalerFunc = context.UnmarshalerFunc
 	// A Handler responds to an HTTP request.
 	// It writes reply headers and data to the Context.ResponseWriter() and then return.
 	// Returning signals that the request is finished;