Add details about event files

[TimManiquet]

I think extra details could be given on the BIDS conversion page on how to turn log files (or else) into event files.

• the onset values need to be first TR-aligned
• it's worth having a condition column in the event files, with contrasts in mind
• events that don't have a duration should become 'n/a', as per BIDS convention

I assign myself but do check if I'm missing anything

[costantinoai]

Good one. This should also be mentioned in #100