chore: codecov job and update readme with codecov badge

Author: softvar

.github/workflows/test.yml

@@ -33,3 +33,10 @@ jobs:
 
     - name: Run tests
       run: coverage run --source=json2html setup.py test
+
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v3
+      env:
+        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+      with:
+        fail_ci_if_error: true

README.md

@@ -2,7 +2,7 @@
 
 Python module to convert `JSON` into a human readable `HTML Table` representation.
 
-![Latest Version](https://img.shields.io/pypi/v/json2html.svg) ![Downloads](https://img.shields.io/pypi/dm/json2html.svg) [![CI](https://github.com/softvar/json2html/workflows/CI/badge.svg?branch=master)](https://github.com/softvar/json2html/actions?query=workflow%3ACI)
+![Latest Version](https://img.shields.io/pypi/v/json2html.svg) ![Downloads](https://img.shields.io/pypi/dm/json2html.svg) [![CI](https://github.com/softvar/json2html/workflows/CI/badge.svg?branch=master)](https://github.com/softvar/json2html/actions?query=workflow%3ACI) [![codecov](https://codecov.io/gh/softvar/json2html/branch/master/graph/badge.svg?token=)](https://codecov.io/gh/softvar/json2html)
 
 ## Features