Add a runnable example

example/pure_zstd_example.dart

@@ -0,0 +1,29 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:pure_zstd/pure_zstd.dart';
+
+/// `zstd -19` over a file holding `Hello from pure_zstd!`, with a checksum.
+final Uint8List frame = Uint8List.fromList([
+  0x28, 0xb5, 0x2f, 0xfd, 0x24, 0x15, 0xa9, 0x00, 0x00, 0x48, 0x65, 0x6c, //
+  0x6c, 0x6f, 0x20, 0x66, 0x72, 0x6f, 0x6d, 0x20, 0x70, 0x75, 0x72, 0x65,
+  0x5f, 0x7a, 0x73, 0x74, 0x64, 0x21, 0x02, 0x01, 0xd0, 0x63,
+]);
+
+void main() {
+  // One frame: the top-level function is enough.
+  print(utf8.decode(zstdDecode(frame)));
+
+  // Many frames: hold a decoder, which reuses its tables and scratch buffers.
+  final decoder = ZstdDecoder();
+  for (var i = 0; i < 3; i++) {
+    print(utf8.decode(decoder.decode(frame)));
+  }
+
+  // Anything malformed is a ZstdException, never an Error.
+  try {
+    zstdDecode(Uint8List.fromList([...frame, 0x00]));
+  } on ZstdException catch (error) {
+    print(error);
+  }
+}