Update logging documentation to current behaviour.

The README wasn't up to date with current behaviour.

Also, this seems wrong:

```
sam@samtu:~/w/sn/strong-supervisor (feature/log-documentation %) % slr --log=_log ./test/express-app 
strong-agent v0.4.8 profiling app 'express-app' pid '22360'
strong-agent dashboard is at https://strongops.strongloop.com
strong-agent using collector https://collector.strongloop.com:443
ERROR supervisor running without clustering (unsupervised)
express-app listening on http://localhost:3000
strong-agent[22360] connected to collector
sam@samtu:~/w/sn/strong-supervisor (feature/log-documentation %) % find . -name _log
```

Where is the _log file? It is supposed to support the _log even when not detached, right? We need that so we can run undetached from strong-deploy or upstart directly.

Also, the ERROR is new, should not be there. I was writing to stderr so it would be noticed, not to unduly alarm people! Maybe message should go to stdout, now?

Except in above, it should go to _log, what am I missing?

And I think we can actually redirect both process.stdout and process.stderr to our log file, if we want, which would capture the strong-agent messages: and we want them in the log. Either that, or we should be using the strong-agent.profile() `logger` option, so it writes to the log.