Add code example for readme

README.md

@@ -5,5 +5,32 @@
 - [x] Support functions (getConfig, getConfigOrNull, hasConfigKey)
 - [x] Support nullsafety (dart 2.12.2+)
 
+# Example
+```dart
+import 'package:configurable/dotenv_configuration_provider.dart';
+import 'package:configurable/simple_configuration_provider.dart';
+import 'package:configurable/system_config.dart';
+
+void main() {
+  var key = 'app.name';
+  var value = 'CUBETIQ Solution';
+
+  // in-memory provider (built-in)
+  var simpleProvider = SimpleConfigurationProvider();
+  SystemConfig.setProvider(simpleProvider);
+  var result1 = SystemConfig.getOrNull(key, defaultValue: value);
+  // output: CUBETIQ Solution
+  print(result1);
+
+  // dotenv provider (from file .env)
+  var dotenvProvider = DotenvConfigurationProvider();
+  SystemConfig.setProvider(dotenvProvider);
+  var result2 = SystemConfig.getOrNull('HOME');
+
+  // output: user's home directory
+  print(result2);
+}
+```
+
 # Contributors
 - Sambo Chea <sombochea@cubetiqs.com>