Add explanation of what the workflow does

Author: NyanKiyoshi

.github/workflows/test-action.yaml

@@ -1,3 +1,7 @@
+# Tests the GitHub Action (../../action.yaml). This creates annotations,
+# but they won't appear in the PR UI because GitHub only shows annotations
+# that are tied to lines modified in the pull request. Instead, they will
+# be visible in this workflow's logs.
 name: Test GitHub Action
 on:
   pull_request: