signature/policy_config.go
func NewPolicyFromFile(fileName string) (*Policy, error) {
	contents, err := ioutil.ReadFile(fileName)
	if err != nil {
		return nil, err
	}
	policy, err := NewPolicyFromBytes(contents)
	if err != nil {
		return nil, errors.Wrapf(err, "invalid policy in %q", fileName)
	}
	return policy, nil
}
func NewPolicyFromFile(fileName string) (*Policy, error) {
	contents, err := ioutil.ReadFile(fileName)
	if err != nil {
		return nil, err
	}
	policy, err := NewPolicyFromBytes(contents)
	if err != nil {
		return nil, errors.Wrapf(err, "invalid policy in %q", fileName)
	}
	return policy, nil
}
// NewPolicyFromFile returns a policy configured in the specified file.