Add change 05 instructions

how_to/Change_05.md

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+- cat-file: Print hashed objects
+
+This command is the "opposite" of `hash-object`: it can print an object by its
+OID. Its implementation just reads the file at ".ugit/objects/{OID}".
+
+The names `hash-object` and `cat-file` aren't the clearest of names, but they
+are the names that Git uses so we'll stick to them for consistency.
+
+We can now try the full cycle:
+
+```
+$ cd /tmp/new
+$ ugit init
+Initialized empty ugit repository in /tmp/new/.ugit
+$ echo some file > bla
+$ ugit hash-object bla
+0e08b5e8c10abc3e455b75286ba4a1fbd56e18a5
+$ ugit cat-file 0e08b5e8c10abc3e455b75286ba4a1fbd56e18a5
+some file
+```
+
+Note that the name of the file (bla) wasn't preserved as part of this process,
+because, again, the object database is just about storing bytes for later
+retrieval and it doesn't care which filename the bytes came from.