DOC: Add a minimal description of a NiPreps project

[jhlegarreta]

Add a minimal description of a data processing pipeline project under the NiPreps convention.

[jhlegarreta]

This is only the skeleton of the directory structure. The actual cookiecutter content will come after I have better understood the minimum contents.

A few questions/request for feedback:

• Do we need a setup.cfg?
• Should project_slug be inside an src folder? (e.g. smriprep is inside an src folder)
• Should tests be all at the root or inside each module?
• If a testing modules is required outside, should it be called tests or test? (nifreeze has a test folder at the root, but no other *prep has it)

[jhlegarreta]

How should we call a general NiPreps project? I've used processing pipeline, but not totally convinced.