[FIXED] Note in README regarding outbound buffer size
[FIXED] Note in README regarding outbound buffer size

Originally the outbound's buffer size was hardcoded. It is now possible
to set its size with `natsOptions_SetIOBufSize()`, so updating
the note in the README.

Resolves #306

[ci skip]

Signed-off-by: Ivan Kozlovic <a15f8b81a160b4eebe5c84e9e3b65c87b9b2f18e@synadia.com>

nats-io/cnats,nats-io/cnats,nats-io/cnats