Add media/cast/README.md.

Bug: b/162541648
Change-Id: I2ee5c166fb34109fe0fd0470da428b71e836555b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618700
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@{#841743}

media/cast/README.md

+# media/cast/
+
+This directory contains a collection of components related to streaming using
+the Cast Streaming Protocol (over UDP network sockets). They encode/decode raw
+audio or video frames, and send/receive encoded data reliably over a local area
+network.
+
+NOTE: This implementation is **deprecated**, and to be replaced soon by the one
+found in `../../third_party/openscreen/src/cast/streaming/`. Contact
+jophba@chromium.org for details.
+
+# Directory Breakdown
+
+* common/ - Collection of shared utility code and constants.
+
+* logging/ - Packet/Frame logging, for study/experimentation of the protocol at
+  runtime.
+
+* net/ - Wire-level packetization and pacing.
+
+* receiver/ - A minimal receiver implementation, used only for end-to-end
+  testing.
+
+* sender/ - Encoder front-ends and frame-level sender implementation for
+  audio/video.
+
+* test/ - A collection of end-to-end tests, experiments, benchmarks, and related
+  utility code.