[roll third_party/pigweed] docs: Add instructions for obtaining a Gerrit cookie

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48322

third_party/pigweed Rolled-Commits: 99de52de27b9211..89ff5f98f38b1ff
Roller-URL: https://ci.chromium.org/b/8844962605240302912
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8844962605240302912
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I25a68a9074fa005c4dcdcad5b7938d1ea9574bfc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48381
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 99de52d..89ff5f9 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 99de52de27b921179e3dfba2af3837a9b2a11985
+Subproject commit 89ff5f98f38b1ff7a1ff0633c590479e9b592a14