Document new transaction receipts API

[radeksimko]

There's new API endpoint which needs documenting per
https://github.com/monzo/reference-receipts-app/blob/master/main.py#L80-L82
https://github.com/monzo/reference-receipts-app/blob/master/main.py#L122

Related question/request: Do you have any plans on generating this documentation (and potentially SDK for Go and/or other languages) straight from the implementation so it's easier to maintain and always up to date?