Add a warning when RailsTracepointStack is running

[carlosdanielpohlod]

Following feedback is not so intuitive that the logs are saved by default on log/rails_tracepoint_stack.log, so would be useful to have a warning. The message also can advise the high computational resources used to run, like:

"saving the trace to log file log/trace.log ATTENTION: This can make your application slower"

Also, the message should be different if the user sets the logger to have a custom output