Add examples README.md

As mentioned at https://github.com/sdroege/async-tungstenite/pull/3, a later PR could add a brief description to each example.

examples/README.md

@@ -0,0 +1,7 @@
+Examples
+
+* [autobahn-client.rs](https://github.com/sdroege/async-tungstenite/blob/master/examples/autobahn-client.rs)
+* [autobahn-server.rs](https://github.com/sdroege/async-tungstenite/blob/master/examples/autobahn-server.rs)
+* [client.rs](https://github.com/sdroege/async-tungstenite/blob/master/examples/client.rs)
+* [echo-server.rs](https://github.com/sdroege/async-tungstenite/blob/master/examples/echo-server.rs)
+* [server.rs](https://github.com/sdroege/async-tungstenite/blob/master/examples/server.rs)