adding xfail example

--HG--
branch : trunk

doc/example/xfail_demo.py

@@ -0,0 +1,16 @@
+import py
+
+@py.test.mark.xfail
+def test_hello():
+    assert 0
+
+@py.test.mark.xfail(run=False)
+def test_hello2():
+    assert 0
+
+@py.test.mark.xfail("hasattr(os, 'sep')")
+def test_hello3():
+    assert 0
+
+def test_hello5():
+    py.test.xfail("reason")