Add a documentation about boot-time tracing options in
boot config.

Link: http://lkml.kernel.org/r/157867246028.17873.8047384554383977870.stgit@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>