fix(readme): fix broken badge links, add codecov badge
# toml_edit
-[![Build Status](https://img.shields.io/travis/ordian/toml_edit/master.svg?label=linux%20%26%20osx)](
-https://travis-ci.org/ordian/toml_document)
-[![Build Status](https://ci.appveyor.com/api/projects/status/github/ordian/toml_edit?svg=true)](https://ci.appveyor.com/project/ordian/toml-document/branch/master)
+[![Build Status](https://img.shields.io/travis/ordian/toml_edit/master.svg?label=linux%20%26%20osx)](https://travis-ci.org/ordian/toml_edit)
+[![Build Status](https://ci.appveyor.com/api/projects/status/github/ordian/toml_edit?svg=true)](https://ci.appveyor.com/project/ordian/toml-edit/branch/master)
+[![codecov](https://codecov.io/gh/ordian/toml_edit/branch/master/graph/badge.svg)](https://codecov.io/gh/ordian/toml_edit)
[![crates.io](https://img.shields.io/crates/v/toml_edit.svg)](https://crates.io/crates/toml_edit)
[![Join the chat at https://gitter.im/toml_edit/Lobby](https://badges.gitter.im/a.svg)](https://gitter.im/toml_edit/Lobby)
