Merge pull request #2763 from jhamman/docs/skipif_class_warning

add warning to skipping docs re marker inheritance

doc/en/skipping.rst

@@ -118,6 +118,12 @@ You can use the ``skipif`` marker (as any other marker) on classes::
 If the condition is ``True``, this marker will produce a skip result for
 each of the test methods of that class.
 
+.. warning::
+
+   The use of ``skipif`` on classes that use inheritance is strongly
+   discouraged. `A Known bug <https://github.com/pytest-dev/pytest/issues/568>`_
+   in pytest's markers may cause unexpected behavior in super classes.
+
 If you want to skip all test functions of a module, you may use
 the ``pytestmark`` name on the global level:
 
@@ -346,5 +352,3 @@ test instances when using parametrize:
     ])
     def test_increment(n, expected):
         assert n + 1 == expected
-
-