Add an example of sending a message.

Add an example of how to send a message, something like:

```
{ok, D} = irc_client_fsm:start_link("erlirc", "server").
Msg = #irc_cmd{name=privmsg,args=[{message, "Hello!"}],target="Nickname"}.
irc_client_fsm:send(D, Msg).
```
