Add a description of request logger to the documentation

[noritama73]

relate to labstack/echo#2195

Currently, there is no mention of request logger in the document. I suggest that we describe the usage, especially you're thinking that request logger is going mainstream.

For better log monitoring, I suggest that the usage of request logger be written in the on the document.

There is also a problem, should this topic be independent from logger or included in custom logger configuration?

[aldas]

done with #252