Document flaky incompatibility. (#219)

Author: Michael Howitz

README.rst

@@ -190,6 +190,9 @@ Compatibility
 * This plugin may *not* be used with class, module, and package level fixtures.
 * This plugin is *not* compatible with pytest-xdist's --looponfail flag.
 * This plugin is *not* compatible with the core --pdb flag.
+* This plugin is *not* compatible with the plugin
+  `flaky <https://pypi.org/project/flaky/>`_, you can only have
+  ``pytest-rerunfailures`` or ``flaky`` but not both.
 
 Resources
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