Add change 11 instructions

how_to/Change_11.md

@@ -0,0 +1,62 @@
+- write-tree: Write tree objects
+
+Now comes the fun part, where we turn a collection of separate files into a
+single object that represents a directory.
+
+
+The idea is that we will create one additional object that collects all the data
+necessary to store a complete directory. For example, if we have a directory
+with two files:
+```
+$ ls
+cats.txt    dogs.txt
+```
+
+And we want to save the directory, we will first put the individual files into
+the object database:
+```
+$ ugit hash-object cats.txt
+91a7b14a584645c7b995100223e65f8a5a33b707
+$ ugit hash-object dogs.txt
+fa958e0dd2203e9ad56853a3f51e5945dad317a4
+```
+
+Then we will create a "tree" object that has the content of:
+```
+91a7b14a584645c7b995100223e65f8a5a33b707 cats.txt
+fa958e0dd2203e9ad56853a3f51e5945dad317a4 dogs.txt
+```
+
+And we will put this tree object into the object database as well. Then the OID
+of the tree object will actually represent the entire directory! Why? Because we
+can first retrieve the tree object by its OID, then see all the files it
+contains (their names and OIDs) and then read all the OIDs of the files to get
+their actual content.
+
+What if our directory contains other directories? We'll just create tree objects
+for them as well and we'll allow one tree object to point to another:
+```
+$ ls
+cats.txt    dogs.txt    other/
+$ ls other/
+shoes.jpg
+```
+
+The root tree object will look like this:
+```
+blob 91a7b14a584645c7b995100223e65f8a5a33b707 cats.txt
+blob fa958e0dd2203e9ad56853a3f51e5945dad317a4 dogs.txt
+tree 53891a3c27b17e0f8fd96c058f968d19e340428d other
+```
+
+Note that we added a type to each entry so that we know if it's a file or a
+directory. The tree that represents the "other" directory (OID 53891a3c27b17e0f8fd96c058f968d19e340428d) looks like:
+```
+blob 0aa186b09fd81e8cf449ba10eee6aff9711cc1ac shoes.jpg
+```
+We can think about this structure as a tree you know from Computer Science where
+each entries' OID as a pointer to either another tree or to a file (leaf node).
+
+Note that we actually save the tree objects with type "tree" in
+`data.hash_object()` since we don't want the trees to be confused with regular
+files.