Add pandas to the list of libraries not comaptible with pyfakefs

- see pytest-dev#528

docs/usage.rst

@@ -605,6 +605,9 @@ A list of Python modules that are known to not work correctly with
   sufficient demand.
 - the ``Pillow`` image library does not work with pyfakefs at least if writing
   JPEG files (see `this issue <https://github.com/jmcgeheeiv/pyfakefs/issues/529>`__)
+- ``pandas`` (the Python data analysis library) uses its own internal file
+  system access, written in C, and does therefore not work with pyfakefs
+  (see `this issue <https://github.com/jmcgeheeiv/pyfakefs/issues/528>`__)
 
 If you are not sure if a module can be handled, or how to do it, you can
 always write a new issue, of course!