[docs] add justification for SetURLVars and description of alternative approach to setting url vars

Author: Zakariah Chitty

test_helpers.go

@@ -0,0 +1,16 @@
+// Copyright 2012 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mux
+
+import "net/http"
+
+// SetURLVars sets URL variables. This can be used to simplify the testing of
+// request handlers.
+// Alternatively, URL variables can be set by making a route that captures the
+// required variables, starting a server and sending the request to that
+// server.
+func SetURLVars(r *http.Request, val map[string]string) *http.Request {
+	return setVars(r, val)
+}