Add another example to inheritance principle for .json without entities

[yarikoptic]

Upon re-reading current Inheritance principle formulation, nothing seems to
forbid that, and such use in general is great since allows to generalize
common metadata across all files of that datatype.

Notes on possible side-effects from "embracing" such approach (which in
principle I think is not disallowed ATM).

• per rule 4, presence of bold.json forbids presence of another _bold.json
  (i.e with entity) on the same level. So if further specialization e.g. per
  each task- is needed, common metadata needs to be duplicated across them
  (that is what heudiconv does ATM).

  Such restrictions could potentially be elevated if we adopt
  "summarization" refactoring of inheritance principle
  Replace "inheritance" with "summarization" principle bids-2-devel#65
  since order would stop to matter and thus multiple files can apply.

• I think that bids-validators are fine as checked on a single
  ds000248/T1w.json in bids-examples and modified 7t_trt.

• I do not know if tools implement it though but since there was precedence
  for ds000248/T1w.json - they better do ;-)

[effigies]

The legacy validator required the task entity for BOLD files, but the schema validator consistently permits entity-less top-level JSON for any suffix.

[effigies]

To save other reviewers time:

link