Add changes file for optimistic client behavior

changes/optimistic-client

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+  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.