pylint: clarify usage for this package

For people diving into pylint development, add a tip so they don't
get lost hacking on this copy thinking it's the one run by presubmit.

Bug: 866772
Change-Id: Ib1ba7392d110d95b1ebf5e2a0abd2cd1bcabbb1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745817Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686550}

third_party/pylint/README.chromium

@@ -8,6 +8,14 @@ Security Critical: No
 Description:
 This directory contains the pylint module.
 
+This code is not included in the final build.  It is only used to run linting
+checks on code (usually at presubmit time).
+
+This is only used by code inside the src tree that needs to import the pylint
+module directly.  If you're using `pylint` to check your code (e.g. via the
+PRESUBMIT.cfg file), then this copy is *not* used.  That pylint comes from the
+depot_tools repo instead.
+
 Local Modifications:
 - applied upstream fix https://bitbucket.org/logilab/pylint/commits/5df347467ee0
 - applied fix to work around bad interaction between sys.path manipulation in