Add information about GBP in header

src/currencyExchange.h

@@ -1,5 +1,7 @@
 #include <stdio.h>
 
 #define CURRENT_USD_RATE_OF_ONE_EURO 1.07
+#define CURRENT_GBP_RATE_OF_ONE_EURO 0.89
 
 float toUSD(float euro);
+float toGBP(float euro);