Fix handling of README files with extensions

[thht]

PR Description

According to the BIDS specs, a README file can have no extension or one of the following: .txt, .md, .rst.

The current behavior of MNE-BIDS is to always assume an extensionless README to be present. A README file with one of the valid extensions is simply ignored, a new README file is written. This results in two README files being present in the BIDS root folder which violates the specs.

This PR fixes this by looking whether one of the valid extensions are present and uses the correct file.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[sappelhoff]

Thanks! Please ping me once this is ready for review/merge.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.47%. Comparing base (420686e) to head (6935d1e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1318   +/-   ##
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  Coverage   97.46%   97.47%           
=======================================
  Files          40       40           
  Lines        8840     8872   +32     
=======================================
+ Hits         8616     8648   +32     
  Misses        224      224           

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[thht]

pinging @sappelhoff. all checks green. looking forward to your thoughts and input!

[sappelhoff]

LGTM in general, thanks for the contribution.

However I'd also like the behavior to be that when multiple READMEs are discovered, we throw an error, instead of what this PR currently implements (selecting one README, and potentially ignoring the presence of the others)

[thht]

LGTM in general, thanks for the contribution.

However I'd also like the behavior to be that when multiple READMEs are discovered, we throw an error, instead of what this PR currently implements (selecting one README, and potentially ignoring the presence of the others)

Working on it right now ;-)

[thht]

As you can see in my other reply, I added raising a RuntimeError when multiple READMEs are encounted including a test for this.

[sappelhoff]

Thanks @thht!