docs: add badge for codecov

Signed-off-by: deadprogram <ron@hybridgroup.com>

README.md

@@ -3,6 +3,7 @@
 [![GoDoc](https://godoc.org/gobot.io/x/gobot?status.svg)](https://godoc.org/gobot.io/x/gobot)
 [![CircleCI Build status](https://circleci.com/gh/hybridgroup/gobot/tree/dev.svg?style=svg)](https://circleci.com/gh/hybridgroup/gobot/tree/dev)
 [![Appveyor Build status](https://ci.appveyor.com/api/projects/status/ix29evnbdrhkr7ud/branch/dev?svg=true)](https://ci.appveyor.com/project/deadprogram/gobot/branch/dev)
+[![codecov](https://codecov.io/gh/hybridgroup/gobot/branch/dev/graph/badge.svg)](https://codecov.io/gh/hybridgroup/gobot)
 [![Go Report Card](https://goreportcard.com/badge/hybridgroup/gobot)](https://goreportcard.com/report/hybridgroup/gobot)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/hybridgroup/gobot/blob/master/LICENSE.txt)