Add change 08 instructions

how_to/Change_08.md

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+- write-tree: List files
+
+The next command is `write-tree`. This command will take the current working
+directory and store it to the object database. If `hash-object` was for storing
+an individual file, then `write-tree` is for storing a whole directory.
+
+Like `hash-object`, `write-tree` is going to give us an OID after it's done and
+we'll be able to use the OID in order to retrieve the directory at a later time.
+
+In Git's lingo a "tree" means a directory.
+
+We'll get into the details in later changes, in this change we'll only prepare
+the code around the feature:
+
+  + Create a `write-tree` CLI command
+
+  + Create a `write_tree()` function in base module. Why in base module and not
+  in data module? Because `write_tree()` is not going to write to disk directly
+  but use the object database provided by data to store the directory. Hence it
+  belongs to the higher-level base module.
+
+  + Add code to `write_tree()` to print a directory recursively. For now nothing
+  is written anywhere, but we just coded the boilerplate to recursively scan a
+  directory.
+
+We continue in the next change.