Add documentation on how to use ehrql's arrow files in R

[rebkwok]

An ehrql-generated arrow file needs to be read in a specific way in R. We should document this somewhere:

This results in a "not a valid feather file" error.

library(dplyr)

ehrql_data <- feather::read_feather(here::here("output", "dataset.arrow"))

This is fine:

library(arrow)

ehrql_data <- read_feather(here::here("output", "dataset.arrow"))

An ehrql-generated arrow file is also read fine as a feather file by pyarrow:

import pyarrow.feather as ft
ft.read_table("dataset.arrow")

[evansd]

I suspect this is related to the Feather v1 vs v2 formats – this confusion is, I think, why the Feather v2 format ended up being renamed to just Arrow.

The read_feather function from the arrow package can read either format:

read_feather() can read both the Feather Version 1 (V1), a legacy version available starting in 2016, and the Version 2 (V2), which is the Apache Arrow IPC file format. read_ipc_file() is an alias of read_feather().

The feather package itself is abandoned:
https://github.com/wesm/feather

[inglesp]

Add note here.