No info on return values

[faultfactory]

Hello,

I have a minor request. The influx_cpp::builder function returns -1, -2 or -3 depending on certain criteria. I'm not too famliar with influx yet so it would be super helpful if i knew what those meant. Can you add a comment in the code or perhaps a little blurb of text to the ReadMe?

[ILENIMARIUS]

Please put a description on those return codes.

[orca-zhang]

https://github.com/orca-zhang/influxdb-cpp/blob/master/README.md#return-code-cheatsheet