Add README.md to root directory

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Review URL: https://codereview.webrtc.org/1327273002 .

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README.md

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+**WebRTC is a free, open software project** that provides browsers and mobile
+applications with Real-Time Communications (RTC) capabilities via simple APIs.
+The WebRTC components have been optimized to best serve this purpose.
+
+**Our mission:** To enable rich, high-quality RTC applications to be
+developed for the browser, mobile platforms, and IoT devices, and allow them
+all to communicate via a common set of protocols.
+
+The WebRTC initiative is a project supported by Google, Mozilla and Opera,
+amongst others. This page is maintained by the Google Chrome team.
+
+### Development
+
+See http://www.webrtc.org/native-code/development for instructions on how to get
+started developing with the native code.
+
+### More info
+
+ * Official web site: http://www.webrtc.org
+ * Master source code repo: https://chromium.googlesource.com/external/webrtc
+ * Samples and reference apps: https://github.com/webrtc
+ * Mailing list: http://groups.google.com/group/discuss-webrtc
+ * Continuous build: http://build.chromium.org/p/client.webrtc