Document that form submissions data must be compatible with Briefcase format

[matthew-white]

odkmeta assumes that the form submissions data .csv files are compatible with the ODK Briefcase format (the format is described here). Some deviations are acceptable (like extra fields), but many others are not (differences in field names, an absence of KEY/PARENT_KEY, multiple columns for the same select_multiple field, etc.). Many servers within the ODK ecosystem export .csv files in slightly different ways, for example, Ona and KoBo. However, the Briefcase format seems to be the closest thing there is to a standard. odkmeta can't support other formats, so we should document that clearly.

However, note that some servers that export with an unsupported format from within their web interface still support Briefcase. This means that if a user pulls their data using Briefcase rather than from within the web interface, they can still use odkmeta.