Clearly deprecate custom-tabs-client.

This repo has be deprecated for a while, this CL makes it clearer.

Change-Id: I785b8b92a2315bf4e8766212e3e51a3a4104cf54
Reviewed-on: https://chromium-review.googlesource.com/c/custom-tabs-client/+/2254618
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>

README.md

diff --git a/README.md b/README.md
index d59a088..f1c99a1 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,16 @@
+# Deprecated
+
+This repository is deprecated.
+Please use
+[android-browser-helper](https://github.com/GoogleChrome/android-browser-helper)
+or
+[androidx.browser](https://developer.android.com/jetpack/androidx/releases/browser)
+instead.
+
+You can learn more about the deprecation and which library you should use by
+reading
+[Trusted Web Activities, the Lay of the Land](https://developers.google.com/web/android/trusted-web-activity/lay-of-the-land).
+
 # Chrome Custom Tabs - Examples and Documentation
 
 Chrome Custom Tabs provides a way for an application to customize and interact