add message when user is required to touch yubikey to sign. N.B. touch is required during Sign, not SignInit

Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

signer/api/ecdsa_hardware_crypto_service.go

@@ -243,6 +243,7 @@ func sign(ctx *pkcs11.Ctx, session pkcs11.SessionHandle, pkcs11KeyID []byte, pas
 	// Get the SHA256 of the payload
 	digest := sha256.Sum256(payload)
 
+	fmt.Println("Please touch the attached Yubikey to perform signing.")
 	sig, err = ctx.Sign(session, digest[:])
 	if err != nil {
 		logrus.Debugf("Error while signing: %s", err)