docs: added an example of usage

example/main.dart

@@ -0,0 +1,27 @@
+import 'package:coingecko_api/coingecko_api.dart';
+
+Future<void> main() async {
+  print(
+    'This example demonstrates receiving OHLC data (open, high, low, close) '
+    'from CoinGecko on the BTC/USD pair for the last 7 days.',
+  );
+  final api = CoinGeckoApi();
+  print('Calling method getCoinOHLC() ...');
+  final result = await api.coins.getCoinOHLC(
+    id: 'bitcoin',
+    vsCurrency: 'usd',
+    days: 7,
+  );
+  if (!result.isError) {
+    print('getCoinOHLC() results:');
+    result.data.forEach(
+      (item) => print(
+          '${item.timestamp}: open = ${item.open}, high = ${item.high}, low = ${item.low}, close = ${item.close}'),
+    );
+    print('Test method completed successfully.');
+  } else {
+    print('getCoinOHLC() method returned error:');
+    print('${result.errorCode}: ${result.errorMessage}');
+    print('Test method failed.');
+  }
+}