[DOC] Make scope limit more obvious regarding accepting new downloaders

[Remi-Gau]

• Closes none

Changes proposed in this pull request:

• add a warning in the contributing doc regarding adding new downloaders

[github-actions]

👋 @Remi-Gau Thanks for creating a PR!

Until this PR is ready for review, you can include the [WIP] tag in its title, or leave it as a github draft.

Please make sure it is compliant with our contributing guidelines. In particular, be sure it checks the boxes listed below.

• PR has an interpretable title.
• PR links to Github issue with mention Closes #XXXX (see our documentation on PR structure)
• Code is PEP8-compliant (see our documentation on coding style)
• Changelog or what's new entry in doc/changes/latest.rst (see our documentation on PR structure)

For new features:

• There is at least one unit test per new function / class (see our documentation on testing)
• The new feature is demoed in at least one relevant example.

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

We will review it as quick as possible, feel free to ping us with questions if needed.

[Remi-Gau]

After this is in I would close the following as being out of scope:

• [ENH] Request for Inclusion of "KONG Networks" Option in Nilearn Function "nilearn.datasets.fetch_atlas_schaefer_2018()"" #4074
• Add MASSP 2021 Lifespan Probabilistic Atlas [ENH]  #4039
• Add a fetcher for the Melbourne Subcortex Atlas #2273
• Add Gordon et al. 2015 atlas #1167

[codecov]

Codecov Report

Merging #4080 (a9596f2) into main (0fa9ba3) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4080      +/-   ##
==========================================
- Coverage   91.59%   91.51%   -0.09%     
==========================================
  Files         143      143              
  Lines       16076    16076              
  Branches     3339     3339              
==========================================
- Hits        14725    14712      -13     
- Misses        804      819      +15     
+ Partials      547      545       -2     

Flag	Coverage Δ
macos-latest_3.10	91.50% <ø> (?)
macos-latest_3.11	91.50% <ø> (?)
macos-latest_3.12	91.50% <ø> (?)
macos-latest_3.9	91.47% <ø> (?)
ubuntu-latest_3.10	91.50% <ø> (?)
ubuntu-latest_3.11	91.50% <ø> (?)
ubuntu-latest_3.12	91.50% <ø> (ø)
ubuntu-latest_3.9	91.47% <ø> (?)
windows-latest_3.10	91.46% <ø> (ø)
windows-latest_3.11	91.46% <ø> (ø)
windows-latest_3.12	91.46% <ø> (ø)
windows-latest_3.9	91.43% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 5 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[bthirion]

Thx !

[ymzayek]

Thx @Remi-Gau