code: func (c Client) GetExchangeRate(from string, to string) (float64, error) {
  	exchanges, err := c.GetExchangeRates()
  	if err != nil {
  		return 0.0, err
  	}
  	key := from + "_to_" + to
  	if exchanges[key] == "" {
  		return 0.0, errors.New("The exchange rate does not exist for this currency pair")
  	}
  	exchangeFloat, err := strconv.ParseFloat(exchanges[key], 64)
  	if err != nil {
  		return 0.0, err
  	}
  	return exchangeFloat, nil
  }
docs: // GetExchangeRate gets the exchange rate between two specified currencies