update changelog

changelog.md

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+- BREAKING API CHANGE: All WebSocket++ methods now throw an exception of type 
+  `websocketpp::exception` which derives from `std::exception`. This normalizes
+  all exception types under the standard exception hierarchy and allows
+  WebSocket++ exceptions to be caught in the same statement as others. The error
+  code that was previously thrown is wrapped in the exception object and can be
+  accessed via the `websocketpp::exception::code()` method.
 - Feature: Adds `start_perpetual` and `stop_perpetual` methods to asio transport
   These may be used to replace manually managed `asio::io_service::work` objects
 - Feature: Allow setting pong and handshake timeouts at runtime.