[Docs] Add link to View UI in docs/README.md

Change-Id: Ib664751e624c8211035d6b00ff8e9ebc669f05a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585550Reviewed-by: Josh Simmons <jds@google.com>
Commit-Queue: Josh Simmons <jds@google.com>
Auto-Submit: Trevor  Perrier <perrier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835827}

docs/README.md

@@ -123,6 +123,7 @@ used when committed.
     and handle thread safety in Chrome.
 *   [Callback<> and Bind()](callback.md) - All about Callbacks, Closures, and
     Bind().
+*   [Chromium Views UI](ui/index.md) - Working with the desktop UI framework.
 *   [Views Platform Styling](ui/views/platform_style.md) - How views are styled
     to fit in different native platforms
 *   [Tab Helpers](tab_helpers.md) - Using WebContents/WebContentsObserver to add