Explain the stamp (#458)

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

docs/labs/stamp.md

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+# Stamp explanation
+
+This document is for instructors who are using these labs.
+
+The labs provided here are available for anyone.
+You can ask students to do them and submit their completed page.
+When a lab page is completed, a "stamp" is added at the bottom.
+The stamp current has the form `Completed ${resultBeginning} ${hash}`
+where `resultBeginning` is `${timeStamp} ${uuid}`.
+The timestamp is the initial completion time in ISO format, and
+uuid is a random value.
+The hash is a cyrb64Hash hash of `resultBeginning`.
+If two different students have the same stamp, at least one cheated.
+Obviously it's possible for students to cheat by copying the
+correct answers from someone else, but
+some cheaters are lazy.