Add change 06 instructions

how_to/Change_06.md

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+- data: Add types to objects
+
+As we will soon see, there will be different logical types of objects that are
+used in different contexts (even though, from the Object Database's point of
+view, they are just all bytes). In order to lower the chance of using an object
+in the wrong context we're going to add a type tag for each object.
+
+The type is just a string that's going to be prepended to the start of the file,
+followed by a null byte. When reading the file later we'll extract the type and
+verify that it's indeed the expected type.
+
+The default type is going to be `blob`, since by default an object is a
+collection of bytes with no further semantic meaning.
+
+We can also pass `expected=None` to `get_object()` if we don't want to verify
+the type. This is useful for the `cat-file` CLI command which is a debug command
+used for printing all objects.