Document unit test workflow and corpus handling

Add comments to clarify unit test workflow limitations

Author: bact

.github/workflows/unittest.yml

@@ -1,6 +1,13 @@
 # SPDX-FileCopyrightText: 2016-2026 PyThaiNLP Project
 # SPDX-License-Identifier: Apache-2.0
 
+# To reduce test time and save resources,
+# unit tests that download large corpus files will not be covered
+# in this workflow.
+# A separated corpus test workflow is in
+# https://github.com/PyThaiNLP/pythainlp/blob/dev/.github/workflows/corpus.yml
+# which will be triggered only if there is a change in a corpus.
+
 name: Unit test
 
 on: