doc: update migration guide

Signed-off-by: Jian Zeng <[email protected]>

README.md

@@ -89,9 +89,15 @@ For more details of the configuration file, please refer to the [yukid handbook]
 
 ### v0.3.x -> v0.4.x
 
+For configuration:
+
 ```bash
 sed -i.bak 's/log_dir/repo_logs_dir/' /etc/yuki/daemon.toml
 # Also remember to update the `images_upgrade_interval` field in /etc/yuki/daemon.toml if it is set.
 
 sed -i.bak 's/interval/cron/' /path/to/repo/configs/*.yaml
 ```
+
+For post sync hook, the environment variables that are passed to the hook script are changed:
+* `Dir` -> `DIR`: the directory of the repository
+* `Name` -> `NAME`: the name of the repository