Add CHANGELOG entry about whitespace in markers

Author: Perlence

CHANGELOG.rst

@@ -25,6 +25,9 @@ Bug Fixes
   _fixtureinfo attribute (e.g. doctests) (`#2788
   <https://github.com/pytest-dev/pytest/issues/2788>`_)
 
+- Strip whitespace from the markers in INI config (`#2856
+  <https://github.com/pytest-dev/pytest/issues/2856>`_)
+
 
 Improved Documentation
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