crypto/des: add an example to demonstrate EDE2 operation.

EDE2 is a rare DES mode that can be implemented with crypto/des, but
it's somewhat non-obvious so this CL adds an example of doing so.

Fixes #3537.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6721056

src/pkg/crypto/des/des_test.go

@@ -1503,3 +1503,21 @@ func TestSubstitutionTableKnownAnswerDecrypt(t *testing.T) {
 		}
 	}
 }
+
+func ExampleNewTripleDESCipher() {
+	// NewTripleDESCipher can also be used when EDE2 is required by
+	// duplicating the first 8 bytes of the 16-byte key.
+	ede2Key := []byte("example key 1234")
+
+	var tripleDESKey []byte
+	tripleDESKey = append(tripleDESKey, ede2Key[:16]...)
+	tripleDESKey = append(tripleDESKey, ede2Key[:8]...)
+
+	_, err := NewTripleDESCipher(tripleDESKey)
+	if err != nil {
+		panic(err)
+	}
+
+	// See crypto/cipher for how to use a cipher.Block for encryption and
+	// decryption.
+}