DOC: flake8-per-pr for windows users (pandas-dev#23707)

Author: h-vetinari

doc/source/contributing.rst

@@ -591,21 +591,14 @@ run this slightly modified command::
 
    git diff master --name-only -- "*.py" | grep "pandas/" | xargs flake8
 
-Note that on Windows, these commands are unfortunately not possible because
-commands like ``grep`` and ``xargs`` are not available natively. To imitate the
-behavior with the commands above, you should run::
+Windows does not support the ``grep`` and ``xargs`` commands (unless installed
+for example via the `MinGW <http://www.mingw.org/>`__ toolchain), but one can
+imitate the behaviour as follows::
 
-    git diff master --name-only -- "*.py"
+    for /f %i in ('git diff upstream/master --name-only ^| findstr pandas/') do flake8 %i
 
-This will list all of the Python files that have been modified. The only ones
-that matter during linting are any whose directory filepath begins with "pandas."
-For each filepath, copy and paste it after the ``flake8`` command as shown below:
-
-    flake8 <python-filepath>
-
-Alternatively, you can install the ``grep`` and ``xargs`` commands via the
-`MinGW <http://www.mingw.org/>`__ toolchain, and it will allow you to run the
-commands above.
+This will also get all the files being changed by the PR (and within the
+``pandas/`` folder), and run ``flake8`` on them one after the other.
 
 .. _contributing.import-formatting: