re-adding py.test.mark as documented (!) by adding pytest_keyword plugin

--HG--
branch : 1.0.x

doc/test/features.txt

@@ -237,13 +237,15 @@ keyword.
 
 By default, all filename parts and
 class/function names of a test function are put into the set
-of keywords for a given test.  You may specify additional 
+of keywords for a given test.  You can specify additional 
 kewords like this::
 
     @py.test.mark(webtest=True)
     def test_send_http():
         ... 
 
+and then use those keywords to select tests. 
+
 disabling a test class
 ----------------------