Added codecov badge to README

Signed-off-by: Christian Wolf <github@christianwolf.email>

README.md

@@ -5,7 +5,8 @@
     <img src="https://img.shields.io/matrix/nextcloud-cookbook:matrix.org?logo=matrix&label=Join%20the%20discussion&style=flat" alt="Join us on Matrix" >
 </a>
 
-![CI-tests](https://github.com/nextcloud/cookbook/workflows/CI-tests/badge.svg) 
+![CI-tests](https://github.com/nextcloud/cookbook/workflows/CI-tests/badge.svg)
+[![codecov](https://codecov.io/gh/nextcloud/cookbook/branch/master/graph/badge.svg?token=J1DI0KGEX3)](https://codecov.io/gh/nextcloud/cookbook)
 ![GitHub](https://img.shields.io/github/license/nextcloud/cookbook)
 ![GitHub language count](https://img.shields.io/github/languages/count/nextcloud/cookbook)
 ![GitHub Repo stars](https://img.shields.io/github/stars/nextcloud/cookbook?logo=github)
@@ -54,4 +55,4 @@ The parser is far from perfect. If you can help out in any way, please [have a l
 
 #### All of the text is in English?
 This app uses the [Transifex](https://www.transifex.com/nextcloud/nextcloud/cookbook/) translation system.
-You might want to register there to help translating the app to new languages or report errors in existing translations.
+You might want to register there to help translating the app to new languages or report errors in existing translations.