Update depot_tools README cpplint section

Context:
https://chromium-review.googlesource.com/c/infra/infra/+/1422779

Bug: 923438
Change-Id: Ib433e219b45065ae5064aece921e710545e41b59
Reviewed-on: https://chromium-review.googlesource.com/c/1423377
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>

README.md

@@ -72,3 +72,7 @@ https://github.com/google/styleguide/tree/gh-pages/cpplint. Unfortunately, that
 repository is not maintained any more.
 If you want to update `cpplint.py` in `depot_tools`, just upload a patch to do
 so. We will figure out a long-term strategy via issue https://crbug.com/916550.
+
+Note that the `cpplint.py` here is also used by the [Tricium
+analyzer](https://chromium.googlesource.com/infra/infra/+/HEAD/go/src/infra/tricium/functions/cpplint),
+so if the cpplint.py here changes, we should also update the copy used there.