Mention that we don't do cleanup PRs

CONTRIBUTING.md

@@ -31,6 +31,11 b' Some guidelines on contributing to IPython:'
 * New features and backwards-incompatible changes should be documented by adding
   a new file to the [pr](docs/source/whatsnew/pr) directory, see [the README.md
   there](docs/source/whatsnew/pr/README.md) for details.
+* Don't make 'cleanup' pull requests just to change code style.
+  We don't follow any style guide strictly, and we consider formatting changes
+  unnecessary noise.
+  If you're making functional changes, you can clean up the specific pieces of
+  code you're working on.
 
 [Travis](http://travis-ci.org/#!/ipython/ipython) does a pretty good job testing IPython and Pull Requests,
 but it may make sense to manually perform tests (possibly with our `test_pr` script),