Add Pub badge (#96)

diff --git a/packages/file/README.md b/packages/file/README.md
index 9286824..dc050c9 100644
--- a/packages/file/README.md
+++ b/packages/file/README.md
@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/google/file.dart.svg?branch=master)](https://travis-ci.org/google/file.dart)
+[![pub package](https://img.shields.io/pub/v/file.svg)](https://pub.dartlang.org/packages/file)
 
 # File