Add Marketplace v1 badge to README

Fixes #13

README.md

@@ -1,5 +1,7 @@
 # GitHub Action - OPTiMaDe validator
 
+[![GitHub Marketplace](https://img.shields.io/badge/Marketplace-v1-undefined.svg?logo=github&logoColor=white&style=flat)](https://github.com/marketplace/actions/codecov)
+
 This action runs `optimade_validator` from the `optimade` package found in the [`optimade-python-tools` repository](https://github.com/Materials-Consortia/optimade-python-tools) on either a locally running server or a public server.
 
 All minor and patch updates to v1 will be folded into the `v1` tag, so that using the action `@v1` is recommended, since it results in using the latest v1.minor.patch.