net/http: add Handle example

Currently there is no example for http.Handle in the
documentation. This adds an example.

Change-Id: I66ee9983bea1f5237757e1ef4956eae9a056e963
Reviewed-on: https://go-review.googlesource.com/137715
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/example_handle_test.go

+// Copyright 2018 The Go 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 http_test
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"sync"
+)
+
+type countHandler struct {
+	mu sync.Mutex // guards n
+	n  int
+}
+
+func (h *countHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	h.n++
+	fmt.Fprintf(w, "count is %d\n", h.n)
+}
+
+func ExampleHandle() {
+	http.Handle("/count", new(countHandler))
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}