BIDS 1.0.1 added
TotalReadoutTime: defined as the time (in seconds) from the center of the first echo to the center of the last echo (aka “FSL definition” - see here and here how to calculate it). This parameter is required if a corresponding multiple phase encoding directions fieldmap (see 8.9.4) data is present.

• https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/topup/Faq#How_do_I_know_what_phase-encode_vectors_to_put_into_my_--datain_text_file.3F
• http://lcni.uoregon.edu/kb-articles/kb-0003

I wondered how life would be better if sidecar file had it automagically computed for me and BIDS validator shut itself up ;)