Add README file to the ContentCapture component

Change-Id: Iaa88235408b6dee758ca88d638db40a25f74e2fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276751Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784457}

components/content_capture/README.md

+# ContentCapture Component
+
+This directory contains ContentCapture’s renderer and browser parts. They work
+with third_party/blink/renderer/core/content_capture to forward the message
+from blink all the way to the ContentCaptureConsumer on the browser side.
+
+## Classes Overview
+
+ContentCaptureSender has an instance per RenderFrame, it receives the content
+captured/removed messages from its [blink peer](/third_party/blink/renderer/core/content_capture/README.md) and sends them to
+ContentCaptureReceiver on the browser side through the mojom interface.
+
+ContentCaptureReceiver has an instance per RenderFrameHost, it gets the message
+from ContentCaptureSender, then forwards it to ContentCaptureReceiverManager.
+It also generates frame ID which is unique through the browser’s lifecycle, the
+ID is required even if the ContentCaptureReceiver hasn’t received any message
+yet; beside receiving messages, ContentCaptureReceiver can also start or stop
+capturing content on demand.
+
+ContentCaptureReceiverManager has an instance per WebContents and is the base
+class intended to be subclassed by the embedders. It receives messages from
+ContentCaptureReceiver, then attaches the ContentCaptureSession which represents
+its relationship with the main frame, and forwards to the embedders.
+ContentCaptureReceiverManager also works with ContentCaptureController to decide
+whether to start or stop the content capture on navigation starts.
+
+ContentCaptureReceiverManagerAndroid is an Android embedder which is used to
+forward the messages to Java side.
+
+ContentCaptureConsumer is a Java interface via which the implementation receives
+the message that was forwarded by the ContentCaptureReceiverManager.
+
+ContentCaptureConsumerImpl is a ContentCaptureConsumer for Android platform
+service, it converts the received messages to the Android platform format, and
+sends them to the Android Framework though the different tasks.