Begin work on YNAB client

After reading these docs
https://api.youneedabudget.com/v1#/Transactions/createTransaction I successfully
made a request to post a transaction to my YNAB account. Hastily created a
client.go that doesn't contain much at the moment.

monzo_ynab/ynab/client.go

@@ -0,0 +1,24 @@
+package client
+
+import (
+	"serde"
+)
+
+// See requests.txt for more details.
+func PostTransactions(accountID string, txs []serde.Transaction{}) error {
+	return map[string]string{
+		"transactions": [
+			{
+				"account_id": accountID,
+					"date": "2019-12-30",
+					"amount": 10000,
+					"payee_name": "Richard Stallman",
+					"memo": "Not so free software after all...",
+					"cleared": "cleared",
+					"approved": true,
+					"flag_color": "red",
+					"import_id": "xyz-123"
+			}
+		]
+	}
+}