Add change 03 instructions

how_to/Change_03.md

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+- init: Create new .ugit directory
+
+The 'ugit init' command creates a new empty repository.
+
+Git stores all repository data locally, in a subdirectory called ".git", so upon
+initialization we'll create one.
+
+I named the directory ".ugit" rather than ".git" so that it doesn't clash with
+Git, but the idea is the same.
+
+To implement 'init', we could have just called os.makedirs from cli.py, but I
+want to have some separation between different logical parts of the code:
+
+   + cli.py - In charge of parsing and processing user input.
+   + data.py - Manages the data in .ugit directory. Here will be the code that
+   actually touches files on disk.
+
+This separation will be useful as the code will get larger.