Update readme add "level" option

[Henderxx]

Hi everyone, i was going through configuration of rotating files, and in one point i needed to specify few transports but each one for different levels, first i thought it was impossible with rotate file, but in other closed issues i found it as working (#262 ), and it works as expected, but it's not mentioned in readme

[wbt]

Thanks for pointing that out! Would you be interested in making a PR containing text that would be helpful for someone in the situation you were in before figuring that out?

[Henderxx]

i'm not an expert in making PRs but i could try for sure

[wbt]

Thanks for the contribution!