[ENH] do not annex typical top files in BIDS datasets

[Remi-Gau]

context

Mentioned on the matrix datalad channel.

BIDS supports:

• README
• README.txt
• README.rst
• README.md

https://bids-specification.readthedocs.io/en/latest/modality-agnostic-files.html#readme

AIM

This PR aims to make bids config reflect that.

Additional changes

Do not annex license file

• LICENSE

https://bids-specification.readthedocs.io/en/latest/modality-agnostic-files.html#license

Do not annex the bids-validator config

• .bids-validator-config.json

https://github.com/bids-standard/bids-validator#configuration

Even though users may name this file differently, it may make sense to support the default name.

[codecov[bot]]

Codecov Report

Base: 89.61% // Head: 89.61% // No change to project coverage :thumbsup:

Coverage data is based on head (f861d6e) compared to base (5eee418). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #115 +/- ## ======================================= Coverage 89.61% 89.61% ======================================= Files 24 24 Lines 1175 1175 ======================================= Hits 1053 1053 Misses 122 122 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad)

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[mslw]

@Remi-Gau Thanks and apologies that we kept this PR hanging for so long. With 2 approvals and (unrelated) appveyor issues solved all lights are green!

[Remi-Gau]

No worries. I would be total hyprocrit if I complained when others do not merge my stuff right away.