Document the structure of a measure table

[iaindillingham]

We've seen cases where researchers have generated their own measure table for use with deciles-charts (e.g. opensafely/winter-pressures). However, doing so is error-prone because the structure of a measure table is unclear. Consequently, the use of a measure table is also unclear: What columns are required? Are they referenced by name or by index? We should document the structure of a measure table to reduce this confusion.