wallet: add example

wallet/wallet_test.go

@@ -1,6 +1,25 @@
 package wallet
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
+
+func ExampleWallet() {
+	wallet := Wallet{10}
+
+	wallet.Deposit(10)
+	fmt.Printf("The balance is %s\n", wallet.Balance())
+
+	err := wallet.Withdraw(10)
+	if err != nil {
+		fmt.Println(err)
+	}
+	fmt.Printf("The balance is %s\n", wallet.Balance())
+
+	// Output: The balance is 20 BTC
+	// The balance is 10 BTC
+}
 
 func TestWallet(t *testing.T) {
 	t.Run("deposit", func(t *testing.T) {