Add components/mirroring/README.md.

Bug: b/162541648
Change-Id: I057dc7742b7dd8f8e8045484e151106df42f24d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618699
Auto-Submit: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841746}

components/mirroring/README.md

+# //components/mirroring
+
+Service implementation and browser integration for Cast Mirroring.
+
+The Mirroring Service is run in its own sandboxed process. It uses mojo
+message pipes between its process and the privileged browser process to:
+
+ * acquire inputs, such as screen capture video or tab audio capture.
+ * communicate with remote Cast devices using Cast Channel messaging. See `../cast_channel/`.
+ * open UDP network sockets for Cast Streaming packets.
+ * switch between screen mirroring and content remoting modes.
+
+The Service contains all session-management logic, and also interfaces with
+`../../media/cast/` to encode and packetize media in realtime.
+
+Specification: *TODO(jophba): Link to Cast Spec markdown.*
+
+# Directory Breakdown
+
+* browser/ - Browser-side implementation. Also, more can be found in
+  `../../chrome/browser/media/cast_mirroring_service_host.h` and
+  `../../chrome/browser/media/router/providers/cast/`.
+
+* mojom/ - Mojo interfaces.
+
+* service/ - Mirroring Service implementation, as described above.