title: PytestUnknownMarkWarning: Unknown pytest.mark.xxx - is this a typo?
body: <p>I have a file called test.py with the following code:</p>
  
  <pre><code>import pytest
  
  @pytest.mark.webtest
  def test_http_request():
      pass
  
  class TestClass:
      def test_method(self):
          pass
  </code></pre>
  
  <p>pytest -s test.py passed but gave the following warnings:</p>
  
  <pre><code>pytest -s test.py
  =============================== test session starts ============================
  platform linux -- Python 3.7.3, pytest-5.2.4, py-1.8.0, pluggy-0.13.1
  rootdir: /home/user
  collected 2 items
  
  test.py ..
  
  =============================== warnings summary ===============================
  anaconda3/lib/python3.7/site-packages/_pytest/mark/structures.py:325
    ~/anaconda3/lib/python3.7/site-packages/_pytest/mark/structures.py:325:
      PytestUnknownMarkWarning: Unknown pytest.mark.webtest - is this a typo?  You can register
      custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
      PytestUnknownMarkWarning,
  
  -- Docs: https://docs.pytest.org/en/latest/warnings.html
  =============================== 2 passed, 1 warnings in 0.03s ===================
  </code></pre>
  
  <p>Environment: Python 3.7.3, pytest 5.2.4, anaconda3</p>
  
  <p>What is the best way to get rid of the warning message?</p>