docs: document contributor testing workflow

.github/CONTRIBUTING.md

@@ -16,6 +16,71 @@ Follow
 [Google style](https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings)
 for docstrings.
 
+## Testing
+
+When contributing bug fixes or new features, add unit tests to verify the behavior and
+prevent regressions.
+
+### 1. Locate the relevant test file
+
+Test files live in the `tests/` directory and mirror the modules in `src/humanize/`:
+
+- `tests/test_number.py` for number formatting (`intcomma`, `clamp`, `ordinal`, etc.)
+- `tests/test_time.py` for time utilities (`naturaltime`, `naturaldelta`, `naturaldate`,
+  etc.)
+- `tests/test_filesize.py` for file size formatting (`naturalsize`)
+- `tests/test_lists.py` for list formatting (`natural_list`)
+- `tests/test_i18n.py` for internationalization and locale translations
+
+### 2. Follow existing test patterns and use parameterization
+
+Follow `pytest` conventions and use `@pytest.mark.parametrize` when testing multiple
+inputs or boundary conditions.
+
+For example, when testing boundary behavior such as `humanize.clamp()`:
+
+```python
+@pytest.mark.parametrize(
+    ("value", "format", "floor", "ceil", "expected"),
+    [
+        (5, "%s", 0, 10, "5"),
+        (-1, "%s", 0, 10, "0"),
+        (15, "%s", 0, 10, "10"),
+    ],
+)
+def test_clamp_boundaries(value, format, floor, ceil, expected):
+    assert humanize.clamp(value, format=format, floor=floor, ceil=ceil) == expected
+```
+
+### 3. Run focused tests locally
+
+Run only the relevant test file or specific test case while developing:
+
+```sh
+pytest tests/test_number.py
+pytest tests/test_number.py -k test_clamp
+```
+
+### 4. Verify that the test catches regressions
+
+Before finalizing your changes, intentionally introduce a defect or temporarily revert
+your implementation to verify that the new test fails. This confirms that the test
+actively protects against regressions rather than passing vacuously.
+
+### 5. Run the full test suite
+
+Run the complete test suite locally before opening a pull request:
+
+```sh
+pytest
+```
+
+Or using `tox` across supported environments:
+
+```sh
+tox
+```
+
 ## Localization
 
 See [README](https://github.com/python-humanize/humanize#localization).