[Assist Ranker] add readme.

Change-Id: I0ec0f5116d7f4b798e325043b5792a6e85fc3618
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275743Reviewed-by: Charles . <charleszhao@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Auto-Submit: Charles . <charleszhao@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783883}

components/assist_ranker/OWNERS

-amoylan@chromium.org
 charleszhao@chromium.org
 hamelphi@chromium.org
 jiameng@chromium.org

components/assist_ranker/README.md

+# Assist Ranker
+
+[TOC]
+
+## Introduction
+
+Assist Ranker is design to make Chrome smarter by providing client-side machine
+learning (ML) inference in Chrome. It is designed to be a generic infrastructure
+that supports ML needs for all Chrome feature teams on all platforms.
+
+Assist Ranker utilizes UKM logging to log per feature-label events. A ML model
+will be trained in the Cloud based on these logs; and then Assist Ranker will
+download and inference with the model.
+
+It currently only supports Logistic Regression and Multilayer Neural Networks.
+
+Assist Ranker was experiment on ContextualSearch; but it is not used in any
+production projects.
+
+## How to use it
+
+Please contact the owners before you use it.
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