Add directories for extensions + additional demos

demo/README.md

+# Demo application #
+
+This folder contains a demo application that uses ExoPlayer to play a number
+of test streams. It can be used as a starting point or reference project when
+developing other applications that make use of the ExoPlayer library.

demo_misc/README.md

+# Miscellaneous demos #
+
+This folder contains miscellaneous demo applications. For example applications
+that demonstrate use of optional extensions, or more advanced features.
+
+A general purpose ExoPlayer demo application can be found in the [demo](../demo)
+folder.

extensions/README.md

+# Extensions #
+
+This folder contains optional ExoPlayer extensions.