add note to "what's new"

docs/source/whatsnew/pr/no-xdg.rst

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+* Previous versions of IPython on Linux would use the XDG config directory,
+  creating :file:`~/.config/ipython` by default. We have decided to go
+  back to :file:`~/.ipython` for consistency among systems. IPython will
+  issue a warning if it finds the XDG location, and will move it to the new
+  location if there isn't already a directory there.