Add change 09 instructions

how_to/Change_09.md

@@ -0,0 +1,8 @@
+- write-tree: Ignore .ugit files
+
+If we run `ugit write-tree`, we will see that it also prints the content of the
+.ugit directory. This directory isn't part of the user's files, so let's ignore
+it.
+
+Actually, I created a separate `is_ignored()` function. This way if we have any
+other files we want to ignore later we have one place to change.