Add change 12 instructions

how_to/Change_12.md

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+- read-tree: Extract tree from object
+
+This command will take an OID of a tree and extract it to the working directory.
+Kind of the opposite of `write-tree`.
+
+I divided the implementation into a few layers:
+
+`_iter_tree_entries` is a generator that will take an OID of a tree, tokenize it
+line-by-line and yield the raw string values.
+
+`get_tree` uses `_iter_tree_entries` to recursively parse a tree into a
+dictionary.
+
+`read_tree` uses `get_tree` to get the file OIDs and writes them into the
+working directory.
+
+Now we can actually save versions of the working directory! It's nothing like
+proper version control, but we can see that a super basic flow is possible:
+
+  + Imagine you work on some code and you want to save a version.
+  + You run ```ugit write-tree```.
+  + You remember that OID that was printed out (write it on a post-it note or
+  something :)).
+  + Continue working and repeat steps 2 and 3 as necessary.
+  + If you want to return to a previous version, use `ugit read-tree` to restore
+  it to the working directory.
+
+Is it convenient to use? No. But it's just the beginning!