[roll third_party/pigweed] docs: Add mission & philosophies

- First cut at our mission statement
- Sets the stage to gradually add philosophies

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/178910

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9cd274e4314f3e4..a04315b4e778414
Roller-URL: https://ci.chromium.org/b/8764399473575774721
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I78311110b22ba460062e799387a7c7e4d25bd1d1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/180787
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 9cd274e..a04315b 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9cd274e4314f3e4d528ea2e9d8c09c0577a068b7
+Subproject commit a04315b4e77841487f99d99ff3ccd566a96548e3