[roll third_party/pigweed] pw_env_setup: Add README to environment directory

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/80580

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4adef6fa36c9f92..5702eeb4da6f2b3
Roller-URL: https://ci.chromium.org/b/8824470885725561105
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8824470885725561105
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I26d9a56fd61de32b5d1d22a48502a61db23dd7ae
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/79888
Bot-Commit: 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 4adef6f..5702eeb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4adef6fa36c9f9222165e35cadb9e92e3d99477e
+Subproject commit 5702eeb4da6f2b30bd2b75187cecd240b4ee5dc7