crypto/ecdsa: add a package level example

Change-Id: I4063d5ec4ac45561b94472b528583be564981912
Reviewed-on: https://go-review.googlesource.com/120144
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

src/crypto/ecdsa/example_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 ecdsa_test
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"fmt"
+)
+
+func Example() {
+	privateKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		panic(err)
+	}
+
+	msg := "hello, world"
+	hash := sha256.Sum256([]byte(msg))
+
+	r, s, err := ecdsa.Sign(rand.Reader, privateKey, hash[:])
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("signature: (0x%x, 0x%x)\n", r, s)
+
+	valid := ecdsa.Verify(&privateKey.PublicKey, hash[:], r, s)
+	fmt.Println("signature verified:", valid)
+}