Add simpla usage and instructuion on readme

README.md

 # Migration
 
-Useful migration tools
+Useful python migration tools
+
+
+## Requirement
+
+`python-gitlab` requires `Python 3.8+` at least.
+
+Use pip to install the latest stable version of python-gitlab
+> pip install --upgrade python-gitlab 
+
+ 
+
+
+## 1. Group "my_group" on pilot gitlab -> Group "my_group" on new GitLab at KIT
+Usage: migration_group_export_import.py
+
+1. add your personal access token (PAT) getting from the source and target gitlab both. 
+2. In source(pilot) gitlab, find out the group id, which will be migrated. You should access it from the web browser. 
+3. In target(new) gitlab, define the `path`` of your subgroup. (not group name)
+    - `group_namespace_in_target = 'kit/institute/migration'`
+
+In the target gitlab, a group structure "KIT / Institute" has to be eanbled in advance.
+If the last subgroup "migration" is not existing, "migration" will be created first and projects wil be transferred on this subgroup. 
+If the last subgroup "migration" is already existing, then all projects will be transferred to existing subgroup, directly. 
+
+> $ python3 migration_group_export_import.py
+
+Procedure: 
+1. Exporting projects in a group `my_group` from the source gitlab. You needs to know `group_id` for this group `my_group`
+2. Download exported files to the local directory `"{PWD}/exported_files"` 
+3. Importing the exported files to the target gitlab on `"kit/institute/migration"`
+
+ 
+
+## 2. Group "my_group" on pilot gitlab -> Group "my_group" on new GitLab at KIT
+Usage: migration_group_import_only.py
+1. add your personal access token (PAT) getting from the target gitlab only. 
+2. define a set of exported files, that are located under the local directory `"{PWD}/exported_files"`
+3. In target(new) gitlab, define the path of your subgroup
+    - `group_namespace_in_target = 'kit/institute/migration'`
+
+In the target gitlab, a group structure "KIT / Institute" has to be eanbled in advance.
+If the last subgroup "migration" is not existing, "migration" will be created first and projects wil be transferred on this subgroup. 
+If the last subgroup "migration" is already existing, then all projects will be transferred to existing subgroup, directly. 
+
+> $ python3 migration_group_improt_only.py
+
+Procedure: 
+1. No exporting and download is requried. In the directory`"{PWD}/exported_files"` all `*_exported.tar.gz"` files(=projects) are enabled to use.   
+2. Importing the exported files to the target gitlab on the subgroup `"kit/institute/migration"`
+
+
+ 
+
+## 3. Project "my_project" on pilot gitlab -> Project "my_project" on new GitLab at KIT
+Usage: migration_project_export_improt.py
+
+1. add your personal access token (PAT) getting from the source and target gitlab both. 
+2. In source(pilot) gitlab, find the project id or project namespace.
+    - `project_id_in_source = 123456789`          
+    - `source_namespace = 'xx1234/my_project'` 
+3. In target(new) gitlab, define the private namespace. It would be <vornaem.nachname> in general. 
+    - `target_namespace = 'firstname.lastname'`
+
+If project_id_in_source is used, you do not need to defind source_namespace. But if you do not use project_id_in_source, then you have to write the project namespace(path) explicitly.
+For the target namespace, you have to define your private namespace as default, where the project will be transferred. 
+
+> $ python3 migration_group_export_import.py
+
+Procedure:
+1. Exporting a single project from the source gitlab. 
+2. Download exported file to the local directory `"{PWD}/exported_files"`
+3. Importing the exported file to the target gitlab on private namesapce `"firstname.lastname"`
\ No newline at end of file