Documentation of debug mode

[franzholz]

I could never find the debug mode on the online documentation.
I think it should not be placed inside of chapter 2 "Installation and usage".
A new chapter "ERRORs" should be written.

[romm]

Indeed this is probably not the best place where it should be.

What do you think about placing it here:

[franzholz]

There should be a new main topic "errors".