doc: update README.md

Author: skipness

README.md

@@ -1,14 +1,17 @@
 # lzstring
-[![Pub](https://img.shields.io/badge/pub-v1.0.3%2B1-blue.svg)](https://pub.dartlang.org/packages/lzstring)
+[![Pub](https://img.shields.io/badge/pub-v2.0.0%2B2-blue.svg)](https://pub.dartlang.org/packages/lzstring)
 
 Dart implementation of lz-string compression algorithm
 
 The original JavaScript version is [here](https://github.com/pieroxy/lz-string)
 
 ## Usage
 ```dart
-Future<String> compressedString = LZString.compress('Some String');
+Future<String?> compressedString = LZString.compress('Some String');
 Future<String?> decompressedString = LZString.decompress(compressedString);
+
+String? compressedStringSync = LZString.compressSync('Some String');
+String? decompressedStringSync = LZString.decompressSync(compressedStringSync);
 ```
 For more usage, please read the exapmle in [example](https://github.com/skipness/lzstring-dart/tree/master/example) folder