Add change 15 to instructions

how_to/Change_15.md

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+- commit: Record hash of last commit to HEAD
+
+I would like to link new commits to older commits. Right now, if we make changes
+in the working directory and make periodic commits, each commit will be a
+standalone object, separate from all other commits. The motivation for linking
+them together is so that we can look at the commits as a series of snapshots in
+some order.
+
+Before we can do it, let's record the OID of the last commit that we created.
+We'll call the last commit the "HEAD" and just put the OID in .ugit/HEAD file.