Add an example to the readme.

README.md

@@ -2,3 +2,31 @@ rfc6979
 =======
 
 A Go implementation of RFC 6979's deterministic DSA/ECDSA signature scheme.
+
+``` go
+package main
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha512"
+	"fmt"
+	"github.com/codahale/rfc6979"
+)
+
+func main() {
+	// Generate a key pair.
+	// You need a high-quality PRNG for this.
+	k, _ := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+
+	// Hash a message.
+	alg := sha512.New()
+	alg.Write([]byte("I am a potato."))
+	hash := alg.Sum(nil)
+
+	r, s, _ := rfc6979.SignECDSA(k, hash, sha512.New)
+	fmt.Printf("Signature: %X%X", r, s)
+}
+
+```