[codecov] add configuration for codecov.io (#41)

Adding configuration and badge to have code coverage uploaded to
`codecov.io`.

.github/workflows/ci.yaml

@@ -27,7 +27,6 @@ jobs:
         if: always()
         run: cat TestResults.json
       - name: Upload coverage reports to Codecov
-        uses: codecov/codecov-action@v4.0.1
+        uses: codecov/codecov-action@v3
         env:
-          token: ${{ secrets.CODECOV_TOKEN }}
-          slug: cacoco/codemetagenerator
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}

README.md

@@ -1,6 +1,6 @@
 # codemetagenerator
 
-[![GitHub](https://img.shields.io/github/license/cacoco/codemetagenerator)](https://github.com/cacoco/codemetagenerator/blob/main/LICENSE) [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active) [![go ci build](https://github.com/cacoco/codemetagenerator/actions/workflows/ci.yaml/badge.svg)](https://github.com/cacoco/codemetagenerator/actions/workflows/ci.yaml) [![GitHub Release](https://img.shields.io/github/release/cacoco/codemetagenerator?style=flat)](https://github.com/cacoco/codemetagenerator/releases/latest)
+[![GitHub](https://img.shields.io/github/license/cacoco/codemetagenerator)](https://github.com/cacoco/codemetagenerator/blob/main/LICENSE) [![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active) [![go ci build](https://github.com/cacoco/codemetagenerator/actions/workflows/ci.yaml/badge.svg)](https://github.com/cacoco/codemetagenerator/actions/workflows/ci.yaml) [![codecov](https://codecov.io/gh/cacoco/codemetagenerator/graph/badge.svg?token=WML9EOGE4N)](https://codecov.io/gh/cacoco/codemetagenerator) [![GitHub Release](https://img.shields.io/github/release/cacoco/codemetagenerator?style=flat)](https://github.com/cacoco/codemetagenerator/releases/latest) 
 
 This project is a [CodeMeta](https://codemeta.github.io/) project description generator written in [Go](https://go.dev/).