Add change 02 instructions

how_to/Change_02.md

@@ -0,0 +1,7 @@
+- cli: Add argument parser
+
+The real Git executable has multiple sub-commands, like 'git init', 'git commit',
+etc. Let's use Python's built-in argument parser argparse to implement sub-commands.
+
+You can see on the other side which changes were made. Now we can run 'ugit init'
+and see "Hello, World!" printed out.

ugit/cli.py

@@ -1,2 +1,22 @@
+import argparse
+
+
 def main():
+    args = parse_args()
+    args.func(args)
+
+
+def parse_args():
+    parser = argparse.ArgumentParser()
+
+    commands = parser.add_subparsers(dest="command")
+    commands.required = True
+
+    init_parser = commands.add_parser("init")
+    init_parser.set_defaults(func=init)
+
+    return parser.parse_args()
+
+
+def init(args):
     print("Hello, World!")