[Media Router] Add README.md for DIAL discovery

chrome/browser/media/router/discovery/dial/README.md

Index: chrome/browser/media/router/discovery/dial/README.md
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+# DIAL Discovery
+
+Implementation of local network discovery of DIAL devices (Smart TVs, Game
+Consoles, etc.).
+ Devices are exposed through the Media Router for use in the Presentation API.
+
+## Design Docs
+
+* [Dial Discovery design doc](https://docs.google.com/a/chromium.org/document/d/1vLpUgp5mJi6KFaCV3HEMQEZYDKtbcGdwcKNADuzuLzw/edit?usp=sharing)
+* [Media Router design doc](https://www.chromium.org/developers/design-documents/media-router)