README: document a quirk about the dark mode test case

On non-Linux platforms, the prerequisite screenshots will not exist
before the first run of that test case, which will save them (so that
subsequent runs will succeed).

This quirk might surprise developers on platforms other than Linux,
therefore it is a good idea to document it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.gitignore

@@ -13,3 +13,5 @@
 /node_modules/
 /test-results/
 /playwright-report/
+/tests/git-scm.spec.js-snapshots/*-darwin.png
+/tests/git-scm.spec.js-snapshots/*-win32.png

README.md

@@ -189,6 +189,9 @@ $ PLAYWRIGHT_TEST_URL='http://localhost:5000/' npx playwright test --project=fir
 
 For more fine-grained testing, you can pass `-g <regex>` to run only the matching test cases.
 
+> [!NOTE]
+> When running the test suite on platforms other than Linux, the first run will "fail" in the `dark mode` test case. That is expected! This test case relies on previously-generated screenshots that are stored in `tests/git-scm.spec.js-snapshots/`, and for bandwidth reasons only the Linux ones are committed in the Git repository (because they are required to run the PR/CI builds). The first run will store those screenshots so that subsequent runs of this test case will succeed, though.
+
 ## Update manual pages
 
 First, install the Ruby prerequisites: