Document which response codes are logged by `httpstats` (#39)
Document which response codes are logged by `httpstats` (#39)

Took me a bit to realize that some response codes are logged, while other are only logged in debug mode:

```go
// httpevents/request.go
switch {
case is4xx(r.status) || is5xx(r.status):
	l.Log(bytesToStringNonEmpty(fmt), arg...)
default:
	l.Debug(bytesToStringNonEmpty(fmt), arg...)
}
```

This PR just adds a small doc comment to clarify this.
segmentio/events