code: func (pwm *Manager) Save(username, secret string) error {
  	//TODO: username and secret validation
  
  	storedSecret := userSecret{Username: username, Secret: secret}
  
  	_, err := pwm.collection.Upsert(bson.M{"username": username}, storedSecret)
  
  	return err
  }
docs: // Save stores a secret for a specific username.