update readme.md

Author: yuanxiongxiong

README.md

@@ -1,2 +1,9 @@
-# docekr-retag
+# docekr_retag
 docekr retag image without by pull and push
+# Usage
+```python
+from docker_retag import Registry
+registyr_ins = Registry("username", "password", "registry_url", "new_tag", "old_tag")
+
+registyr_ins.add_new_tag_by_registry()
+```