doc: Add coverage badge

README.md

@@ -3,8 +3,7 @@
 [![MIT License](https://img.shields.io/github/license/FortySevenEffects/serde.svg?color=blue)](https://github.com/FortySevenEffects/serde/blob/master/LICENSE)
 ![GitHub tag (latest SemVer)](https://img.shields.io/github/tag/FortySevenEffects/serde.svg?color=blue&label=release)
 [![Travis CI Build](https://img.shields.io/travis/com/FortySevenEffects/serde.svg)](https://travis-ci.com/FortySevenEffects/serde)
-[![Average issue resolution time](https://isitmaintained.com/badge/resolution/FortySevenEffects/serde.svg)](https://isitmaintained.com/project/FortySevenEffects/serde)
-[![Number of open issues](https://isitmaintained.com/badge/open/FortySevenEffects/serde.svg)](https://isitmaintained.com/project/FortySevenEffects/serde)
+[![Coverage Status](https://coveralls.io/repos/github/FortySevenEffects/serde/badge.svg?branch=master)](https://coveralls.io/github/FortySevenEffects/serde?branch=master)
 
 Exchange structured data between Arduino boards.