Add change 13 instructions

how_to/Change_13.md

@@ -0,0 +1,7 @@
+- read-tree: Delete all existing stuff before reading
+
+This is done so that we won't have any old files left around after a read-tree.
+
+Before this change, if we save tree A which contains only `a.txt`, then we save
+tree B which contains `a.txt` and `b.txt` and then we `read-tree` A, we will
+have `b.txt` left over in the working directory.