Add custom currency

// Let's create non-ISO currency, such as bitcoin

// At first, this will throw UnknownCurrencyException
MonetaryAmount moneys = Money.of(new BigDecimal("0.1"), "BTC");

// This happens because bitcoin is unknown to default currency
// providers
System.out.println(Monetary.isCurrencyAvailable("BTC")); // false

// We will build new currency using CurrencyUnitBuilder provided by org.javamoney.moneta
CurrencyUnit bitcoin = CurrencyUnitBuilder
    .of("BTC", "BtcCurrencyProvider") // Set currency code and currency provider name
    .setDefaultFractionDigits(2)      // Set default fraction digits
    .build(true);                     // Build new currency unit. Here 'true' means
                                      // currency unit is to be registered and
                                      // accessible within default monetary context

// Now BTC is available
System.out.println(Monetary.isCurrencyAvailable("BTC")); // True