sha256sum: add example (#14819)

pages/common/sha256sum.md

@@ -26,3 +26,7 @@
 - Only show a message when verification fails, ignoring missing files:
 
 `sha256sum --ignore-missing --check --quiet {{path/to/file.sha256}}`
+
+- Check known SHA256 sum of a file:
+
+`echo {{known-sha256-sum-of-the-file}} {{path/to/file}} | sha256sum --check`