Add an example of using HTTPRequest to contact a REST API

We already had an example to download and display an image,
but having another example dedicated to REST APIs doesn't hurt.

doc/classes/HTTPRequest.xml

@@ -6,6 +6,27 @@
 	<description>
 		A node with the ability to send HTTP requests. Uses [HTTPClient] internally.
 		Can be used to make HTTP requests, i.e. download or upload files or web content via HTTP.
+		[b]Example of contacting a REST API and printing one of its returned fields:[/b]
+		[codeblock]
+		func _ready():
+		    # Create an HTTP request node and connect its completion signal.
+		    var http_request = HTTPRequest.new()
+		    add_child(http_request)
+		    http_request.connect("request_completed", self, "_http_request_completed")
+
+		    # Perform the HTTP request. The URL below returns some JSON as of writing.
+		    var error = http_request.request("https://httpbin.org/get")
+		    if error != OK:
+		        push_error("An error occurred in the HTTP request.")
+
+
+		# Called when the HTTP request is completed.
+		func _http_request_completed(result, response_code, headers, body):
+		    var response = parse_json(body.get_string_from_utf8())
+
+		    # Will print the user agent string used by the HTTPRequest node (as recognized by httpbin.org).
+		    print(response.headers["User-Agent"])
+		[/codeblock]
 		[b]Example of loading and displaying an image using HTTPRequest:[/b]
 		[codeblock]
 		func _ready():