[Media Router] Add README.md for DIAL discovery

BUG=687375

Review-Url: https://codereview.chromium.org/2844103002
Cr-Commit-Position: refs/heads/master@{#467918}

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

+# 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)