custom format documentation

[TheunisKotze]

I was struggling to import a custom csv using the "generic format" as specified on the prepare screen. It would simply log an error in console. Had a quick look at the code and it seems you need to add a header line "#custom". At least, that makes it work. Adding that to the docs might save someone 30 mins

[cojapacze]

Example has been updated. Thank you for reporting.