Add 'Update Mechanism: Manual' to README.chromium files

Bug: 445311061
Change-Id: I2b756ad5795b793be54855c8e1883a2a2d8bbefd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6953660
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Auto-Submit: Jordan Brown <rop@google.com>
Reviewed-by: Rachael Newitt <renewitt@google.com>
Commit-Queue: Jordan Brown <rop@google.com>
Owners-Override: Jordan Brown <rop@google.com>
Cr-Commit-Position: refs/heads/main@{#1516347}
NOKEYCHECK=True
GitOrigin-RevId: 305b2fe1fa7a2c66e0ffee71ff6cdab160c570a8

README.chromium

diff --git a/README.chromium b/README.chromium
index facc7c5..7cd5806 100644
--- a/README.chromium
+++ b/README.chromium
@@ -3,6 +3,7 @@
 URL: https://github.com/benjaminp/six
 Version: 1.14.0
 Revision: 3a3db7510b33eb22c63ad94bc735a9032949249f
+Update Mechanism: Manual
 License: MIT
 License File: LICENSE
 Security Critical: No