tracing: Update debugfs README file

Update the README file in debugfs/tracing to something more useful.
What's currently in the file is very old and what it shows doesn't
have much use. Heck, it tells you how to mount debugfs! But to read
this file you would have already needed to mount it.

Replace the file with current up-to-date information. It's rather
limited, but what do you expect from a pseudo README file.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>