docs: add singleton usage examples

- Update README.md with singleton usage examples
- Add new file examples/singleton_usage.py
- Remove setup.cfg

This commit enhances the documentation by adding detailed examples of singleton usage in the README.md and includes a new file with additional singleton examples. The removal of setup.cfg is part of a configuration cleanup.

README.md

@@ -40,18 +40,6 @@ conda activate zeeland
 pip install poetry
 ```
 
-Then you can run the client using the following command:
-
-```bash
-zeeland --help
-```
-
-or with `Poetry`:
-
-```bash
-poetry run zeeland --help
-```
-
 ### Basic Usage
 
 Create a metadata file in the default storage path.
@@ -82,6 +70,37 @@ if __name__ == "__main__":
     main()
 ```
 
+The metadata file will be saved in the default storage path, which is `~/.zeeland/test/metadata.json`.
+
+Singleton usage
+
+```python
+from zeeland import Singleton, singleton
+
+
+@singleton()
+class TestSingleton:
+    pass
+
+
+instance1 = TestSingleton()
+instance2 = TestSingleton()
+
+assert instance1 is instance2
+
+
+class TestSingletonWithArgs(metaclass=Singleton):
+    def __init__(self, value):
+        self.value = value
+
+
+instance1 = TestSingletonWithArgs("test1")
+instance2 = TestSingletonWithArgs("test2")
+
+assert instance1 is instance2
+assert instance1.value == "test1"
+```
+
 ### Makefile usage
 
 [`Makefile`](https://github.com/Undertone0809/zeeland/blob/main/Makefile) contains a lot of functions for faster development.

examples/singleton_usage.py

@@ -0,0 +1,24 @@
+from zeeland import Singleton, singleton
+
+
+@singleton()
+class TestSingleton:
+    pass
+
+
+instance1 = TestSingleton()
+instance2 = TestSingleton()
+
+assert instance1 is instance2
+
+
+class TestSingletonWithArgs(metaclass=Singleton):
+    def __init__(self, value):
+        self.value = value
+
+
+instance1 = TestSingletonWithArgs("test1")
+instance2 = TestSingletonWithArgs("test2")
+
+assert instance1 is instance2
+assert instance1.value == "test1"