Add more context about updater to the README.md file.

Change-Id: Ie7cbfe1ef76d0cbb26535748488fd4a2c3c1b1a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065396
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743171}

chrome/updater/README.md

-This is the code for the client updater that will soon be used by
-desktop Chrome, on macOS and Windows.
+An updater for desktop client software using Chromium code and tools.
 
-Please join chrome-updates-dev@chromium.org for topics related to this project.
+* The code lives in //chrome/updater.
+* Design Doc: http://bit.ly/chromium-updater
+* Please join chrome-updates-dev@chromium.org for topics related to the project.
+
+The updater will be built from a common, platform neutral code base, as part of
+the Chrome build. The updater is going to be a drop-in replacement for Google
+Update/Omaha/Keystone and could be customized by 3rd party embedders to for
+updating non-Google client software, such as Edge.
+
+The desktop platforms include Windows, macOS, Linux.
+
+There are many reasons to start a new code base for this:
+
+* Reducing the development cost and the code duplication among platforms.
+* Implementing update algorithms consistently and correctly: checking for
+updates, applying updates, gathering metrics, and load shedding.
+* Use world-class developer tool chains for build, security, and stability.
+
+The existing Omaha/Keystone design, implementation, and production issues apply
+to this project as well.