Add visible warning about subdirs

[gdalle]

In CompatHelper PR's, I have often had the case where

• a package's compat is bumped in Project.toml
• it stays the same in test/Project.toml because CompatHelper.main() does not include subdirs = ["", "test", "docs"]
• the latest version is not therefore tested in the PR due to test compat bounds
• the check still ends up green so the user may never know

This PR tries to correct course by making it more explicit that subdirs is among the most important options to customize in a realistic package. It is important because CompatHelper itself doesn't have a test/Project.toml and so we cannot directly demonstrate that inside its CompatHelper.yml file (although we could add docs).

Related:

• Check docs/Project.toml and test/Project.toml as well #454
• Merge all PRs for the subdirs into a single PR #516
• Does CompatHelper send PRs to update project.toml files in subdirectories? #408

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1a3b572) to head (3c590a4).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #533   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          406       403    -3     
=========================================
- Hits           406       403    -3     

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