Add change 16 to instructions

how_to/Change_16.md

@@ -0,0 +1,23 @@
+- commit: set parent to HEAD
+
+When creating a new commit, we will use the HEAD to link the new commit to the
+previous commit. We'll call the previous commit the "parent commit" and we will
+save its OID in the "parent" key on the commit object.
+
+For example, HEAD is currently bd0de093f1a0f90f54913d694a11cccf450bd990 and we
+create a new commit, the new commit will look like this in the object store:
+
+```
+tree 50bed982245cd21e2798f179e0b032904398485b
+parent bd0de093f1a0f90f54913d694a11cccf450bd990
+
+This is the commit message!
+```
+
+The first commit in the repository will obviously have no parent.
+
+Now we can retrieve the entire list of commits just by referencing the last
+commit! We can start from the HEAD, read the "parent" key on the HEAD commit and
+discover the commit before HEAD. Then read the parent of that commit, and go
+back on and on... This is basically a linked list implemented over the object
+database.