sha*sum: add example about the creation of a list of checksums

pages/common/sha224sum.md

@@ -10,6 +10,10 @@
 
 `sha224sum {{filename1}} {{filename2}}`
 
+- Calculate and save the list of SHA224 checksums to a file
+
+`sha256sum {{filename1}} {{filename2}} > {{filename.sha224}}`
+
 - Read a file of SHA224 sums and verify all files have matching checksums:
 
 `sha224sum --check {{filename.sha224}}`