singleflight: Add an example

I was curious about the best way to initialize a Group - it turns out you just
do `var g Group` - but figured this package could use a package-level example
demonstrating an example use case.

singleflight/example_test.go

@@ -0,0 +1,45 @@
+/*
+Copyright 2018 The Go Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package singleflight_test
+
+import (
+	"fmt"
+	"log"
+	"strconv"
+	"time"
+
+	"github.com/golang/groupcache/singleflight"
+)
+
+var counter int32
+var group singleflight.Group
+
+func Example() {
+	// No matter how many goroutines call Do, only one call per key will be in
+	// progress at any time. Callers that share a key will get the same results
+	// as an in-flight call with that key.
+	n := int32(41)
+	v, err := group.Do(strconv.FormatInt(int64(n), 10), func() (interface{}, error) {
+		time.Sleep(time.Millisecond) // simulate time-consuming action
+		return n + 1, nil
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(v.(int32))
+	// Output: 42
+}