docs(CHANGES): Add entry for #510 file protocol fix

why: Document the pytest plugin fix for strict build environments
what:
- Add Tests section for 0.39.x release
- Note the protocol.file.allow=always addition to gitconfig fixture

Author: tony

CHANGES

@@ -20,6 +20,14 @@ $ uv add libvcs --prerelease allow
 _Notes on the upcoming release will go here._
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Tests
+
+- pytest plugin: Add `protocol.file.allow=always` to `gitconfig` fixture (#510)
+
+  Fixes #509: "transport 'file' not allowed" errors in strict build
+  environments (e.g., Arch Linux packaging) where git submodule operations
+  spawn child processes that don't inherit local repo config.
+
 <!-- Maintainers, insert changes / features for the next release here -->
 
 ## libvcs 0.38.2 (2026-01-24)