Add link to Chromium UI docs to main doc index.

Bug: none
Change-Id: I897f646c41b89dbf8343c63882c787ec6dfd5114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4114461
Auto-Submit: Peter Kasting <[email protected]>
Reviewed-by: Robert Liao <[email protected]>
Commit-Queue: Robert Liao <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1084614}
diff --git a/docs/README.md b/docs/README.md
index 7613944..54c133f 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -428,6 +428,9 @@
 *   [Chrome Speed](speed/README.md) - Documentation for performance measurements and regressions in Chrome.
 *   [Chrome Speed Metrics](speed_metrics/README.md) - Documentation about user experience metrics on the web and their JavaScript APIs.
 
+### UI
+*   [Chromium UI Platform](ui/index.md) - All things user interface
+
 ### Probably Obsolete
 *   [TPM Quick Reference](tpm_quick_ref.md) - Trusted Platform Module notes.
 *   [System Hardening Features](system_hardening_features.md) - A list of