Update README.md

Author: mduesterhoeft

README.md

@@ -1,5 +1,6 @@
 [![](https://jitpack.io/v/moia-dev/lambda-kotlin-request-router.svg)](https://jitpack.io/#moia-dev/lambda-kotlin-request-router)
 [![CircleCI](https://circleci.com/gh/moia-dev/lambda-kotlin-request-router.svg?style=svg&circle-token=c7f438800605a6528b630d44dfa8a1da3ec3c06a)](https://circleci.com/gh/moia-dev/lambda-kotlin-request-router)
+[![codecov](https://codecov.io/gh/moia-dev/lambda-kotlin-request-router/branch/master/graph/badge.svg)](https://codecov.io/gh/moia-dev/lambda-kotlin-request-router)
 
 # lambda-kotlin-request-router