Add coverage badge to README

Author: brianegan

CHANGELOG.md

@@ -5,6 +5,7 @@
   * Enforce Pedantic package lint rules
   * Enforce `public_member_api_docs` lint rule and add docs to missing parts
   * Update `travis.yml` with support for dartfmt, analysis, and code coverage
+  * Add coverage badge to root README
   * Pub.dev updates
     * Longer description
     * https for package url

README.md

@@ -1,6 +1,7 @@
 # redux.dart
 
 [![Build Status](https://api.travis-ci.org/johnpryan/redux.dart.svg?branch=master)](https://travis-ci.org/johnpryan/redux.dart)
+[![codecov](https://codecov.io/gh/johnpryan/redux.dart/branch/master/graph/badge.svg)](https://codecov.io/gh/johnpryan/redux.dart)
 
 [Redux](http://redux.js.org/) for Dart using generics for typed State. It includes a rich ecosystem of [Docs](#docs), [Middleware](#middleware), [Dev Tools](#dev-tools) and can be combined with Flutter using the [flutter_redux](https://pub.dartlang.org/packages/flutter_redux) package.