Add changes file for optimistic client behavior

  o Major features:

    - When using an exit nodes running 0.2.3.1-alpha and later,

      clients can now "optimistically" send data on a stream before

      the exit node reports that the stream has opened. This can save

      a round trip when starting connections with protocols where the

      client speaks first.  Implements proposal 181; code by Ian

      Goldberg.