[roll third_party/pigweed] docs: Add changelog update instructions and tool

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/181765

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: afac8b29114e86e..89588329bfc42c9
Roller-URL: https://ci.chromium.org/b/8763241307302201201
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8c874b8b8e23818b337ea819801678e70e987fc1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/182686
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index afac8b2..8958832 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit afac8b29114e86e78a1e38d3360ffcef50eaf2f7
+Subproject commit 89588329bfc42c935cb1c77e996a495700e62eda