A single transaction can be added by PUT to /add.
(I read that PUT, not POST, should be used to create;
perhaps the web add form should also use PUT ?)
As with the web form, the `add` capability is required (and enabled by
default).
Here's how to test with curl:
$ curl -s http://127.0.0.1:5000/add -X PUT -H 'Content-Type: application/json' --data-binary @in.json; echo
New readJsonFile/writeJsonFile helpers in Hledger.Web.Json
are handy for generating test data. Eg:
>>> writeJsonFile "in.json" (head $ jtxns samplejournal)
