docs: update the example structures to have session sub-directory

[kcho]

Hi @tashrifbillah,

I have been thinking that the example BIDS structures in the README.md may confuse some of the new people trying to run luigi-pipeline. If we would like to force the structure to have the ses-* directories below each subject directory, we may want to update the README as well.

(+ I don't have permission to create the branch here, so PRing using fork again)

[tashrifbillah]

@eknyazhe and @yrathi , pointed out one important thing--reviewing the code base, I realized ses-*/ is not required in the template, only sub-* is.

[tashrifbillah]

I can introduce this flexibility at my end. Regardless, we can merge your PR.

[tashrifbillah]

Hi Kevin,

I don't think a ses-* directory is mandatory:

from https://bids.neuroimaging.io/