Provide an example with "Sign Regexp" input

It might be even more useful to list the available sign names here.
Fix off-by-one error
Display debug message, if quickfixsigns_max is reached

Ref: https://github.com/tomtom/quickfixsigns_vim/issues/38
Merge branch 'debug-info-with-max-reached' of https://github.com/blueyed/quickfixsigns_vim