Update documentation to indicate all fields need to be present in CSV

[peterdesmet]

Camtrap DP relies on Data Package v1, which requires the CSV files to have all the fields in the order they are defined in the table schema. This limitation is lifted in Data Package v2 with fieldMatch, but until we make use of v2, which should probably indicate in https://github.com/tdwg/camtrap-dp/blob/main/pages/data.md that this is currently a requirement.

See also #172