Add doc on desktop input event routing

A comment-friendly doc version is available at
http://doc/1jBT-aqlwb4I7_OH6xC4yX5sn8zlZTOXbfYiU4coG0yQ

Bug: None
Change-Id: I135f0ba144c437af6bd366625e8cc989c246d7a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571803
Reviewed-by: Robert Liao <[email protected]>
Commit-Queue: Robert Liao <[email protected]>
Cr-Commit-Position: refs/heads/master@{#833769}
diff --git a/docs/ui/index.md b/docs/ui/index.md
index 749ab49..e49d6a6 100644
--- a/docs/ui/index.md
+++ b/docs/ui/index.md
@@ -20,6 +20,7 @@
 * [Platform Styling](/docs/ui/views/platform_style.md)
 * [Product Excellence](/docs/ui/product_excellence/index.md)
 * [UI Devtools](/docs/ui/ui_devtools/index.md)
+* [Input Event Routing](/docs/ui/input_event/index.md)
 
 Archival Documentation on Chrome UI.
 * [Aura](/docs/ui/aura/index.md)