Add testdir examples to CONTRIBUTING guide

Hopefully Closes: #4151

CONTRIBUTING.rst

@@ -280,6 +280,37 @@ Here is a simple overview, with pytest-specific bits:
     base: features        # if it's a feature
 
 
+Writing Tests
+----------------------------
+
+Writing tests for plugins or for pytest itself is done using the `testdir fixture <https://docs.pytest.org/en/latest/reference.html#testdir>`_,
+
+For example:
+
+.. code-block:: python
+
+    def test_true_assertion(testdir):
+        testdir.makepyfile(
+            """
+            def test_foo():
+                assert True
+        """
+        )
+        result = testdir.runpytest()
+        result.assert_outcomes(failed=0, passed=1)
+
+
+    def test_true_assertion(testdir):
+        testdir.makepyfile(
+            """
+            def test_foo():
+                assert False
+        """
+        )
+        result = testdir.runpytest()
+        result.assert_outcomes(failed=1, passed=0)
+
+
 Joining the Development Team
 ----------------------------