ci: add ddt to the tests requirements and update the contributing file accordingly

.github/workflows/ci.yaml

@@ -45,7 +45,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - name: Append GitHub Actions system path
         run: echo "$HOME/.local/bin" >> $GITHUB_PATH
-      - run: pip install coveralls
+      - run: pip install coveralls ddt
       - run: pip install .
       - run: coverage run --source=yamllint -m unittest discover
       - name: Coveralls

CONTRIBUTING.rst

@@ -14,9 +14,12 @@ Pull Request Process
    .. code:: bash
 
     pip install --user .
-    pip install ddt
-    python -m unittest discover  # all tests...
-    python -m unittest tests/rules/test_commas.py  # or just some tests (faster)
+    pip install coveralls ddt
+    # all tests...
+    python -m coverage run --source=yamllint -m unittest discover
+    coverage report
+    # or just some tests (faster)
+    python -m unittest tests/rules/test_commas.py
 
 3. If you add code that should be tested, add tests.