Update getting-started.rst

From the description it seemed to me as if just prefixing your methods with ``test_`` was enough, but you also need to prefix your class with ``Test``. Of course, in the reference material this is clearly stated, but I think it makes sense to mention it here as well, since you also mention the part about the methods' prefix.

doc/en/getting-started.rst

@@ -127,7 +127,7 @@ Once you develop multiple tests, you may want to group them into a class. pytest
             x = "hello"
             assert hasattr(x, "check")
 
-``pytest`` discovers all tests following its :ref:`Conventions for Python test discovery <test discovery>`, so it finds both ``test_`` prefixed functions. There is no need to subclass anything. We can simply run the module by passing its filename:
+``pytest`` discovers all tests following its :ref:`Conventions for Python test discovery <test discovery>`, so it finds both ``test_`` prefixed functions. There is no need to subclass anything, but make sure to prefix your class with ``Test`` otherwise the class will be skipped. We can simply run the module by passing its filename:
 
 .. code-block:: pytest