Improve docs a bit

CONTRIBUTING.rst

@@ -283,9 +283,9 @@ Here is a simple overview, with pytest-specific bits:
 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>`_,
+Writing tests for plugins or for pytest itself is often done using the `testdir fixture <https://docs.pytest.org/en/latest/reference.html#testdir>`_, as a "black-box" test.
 
-For example:
+For example, to ensure a simple test passes you can write:
 
 .. code-block:: python
 
@@ -300,6 +300,11 @@ For example:
         result.assert_outcomes(failed=0, passed=1)
 
 
+Alternatively, it is possible to make checks based on the actual output of the termal using
+*glob-like* expressions:
+
+.. code-block:: python
+
     def test_true_assertion(testdir):
         testdir.makepyfile(
             """
@@ -308,7 +313,12 @@ For example:
         """
         )
         result = testdir.runpytest()
-        result.assert_outcomes(failed=1, passed=0)
+        result.stdout.fnmatch_lines(["*assert False*, "*1 failed*"])
+        
+When choosing a file where to write a new test, take a look at the existing files and see if there's
+one file which looks like a good fit. For example, a regression test about a bug in the ``--lf`` option
+should go into ``test_cacheprovider.py``, given that this option is implemented in ``cacheprovider.py``.
+If in doubt, go ahead and open a PR with your best guess and we can discuss this over the code.
 
 
 Joining the Development Team