Explain better SDC and B0FieldSource requirement.

[toomanycats]

Changes proposed in this pull request

Explain that sdcflows has it's own code base but it's still included in fmriprep.
I'm actually unsure about this but I think this is correct: Tell users to add the B0FieldSource key and value to all fieldmap images.

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[oesteban]

I just noted down a couple of things. @effigies can we maybe make sure this gets merged soon-ish?