Add more comments to the code

Signed-off-by: Songmin Li <lisongmin@protonmail.com>

podman_compose.py

@@ -268,6 +268,8 @@ def rec_subs(value, subs_dict):
             # Load service's environment variables
             subs_dict = subs_dict.copy()
             svc_envs = {k: v for k, v in value['environment'].items() if k not in subs_dict}
+            # we need to add `svc_envs` to the `subs_dict` so that it can evaluate the
+            # service environment that reference to another service environment.
             subs_dict.update(svc_envs)
             svc_envs = rec_subs(svc_envs, subs_dict)
             subs_dict.update(svc_envs)