add codecov badge

Author: hnvn

Diff for: README.md

@@ -1,6 +1,6 @@
 # PatternFormatter
 
-[![pub package](https://img.shields.io/pub/v/pattern_formatter.svg)](https://pub.dartlang.org/packages/pattern_formatter) ![](https://github.com/hnvn/flutter_pattern_formatter/workflows/unit%20test/badge.svg)
+[![pub package](https://img.shields.io/pub/v/pattern_formatter.svg)](https://pub.dartlang.org/packages/pattern_formatter) ![](https://github.com/hnvn/flutter_pattern_formatter/workflows/unit%20test/badge.svg) [![codecov](https://codecov.io/gh/hnvn/flutter_pattern_formatter/branch/master/graph/badge.svg)](https://codecov.io/gh/hnvn/flutter_pattern_formatter)
 
 A Flutter package provides some implementations of TextInputFormatter that format input with pre-defined patterns