Documentations for 3xx and 4xx is needed. 

Hi, 

I have been using yagg for generating the API documentation. I found API are generating only for success cases.  I have found this piece of code.  Here you are allowing only for status codes ranges in 2XX.  

```
func IsStatusCodeValid(code int) bool  {
	if code >= 200 && code < 300 {
		return true
	} else {
		return false
	}
}
```

please rewrite it to allow 3xx and 4xx
