tracing: Add error_log to README

Add brief blurb about error_log to the 'Important files' section.

Link: http://lkml.kernel.org/r/c81e60f9aded495081231a32d2d1023c4d043a7a.1554072478.git.tom.zanussi@linux.intel.com

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -4702,6 +4702,7 @@ static const char readme_msg[] =
 	"  trace_pipe\t\t- A consuming read to see the contents of the buffer\n"
 	"  current_tracer\t- function and latency tracers\n"
 	"  available_tracers\t- list of configured tracers for current_tracer\n"
+	"  error_log\t- error log for failed commands (that support it)\n"
 	"  buffer_size_kb\t- view and modify size of per cpu buffer\n"
 	"  buffer_total_size_kb  - view total size of all cpu buffers\n\n"
 	"  trace_clock\t\t-change the clock used to order events\n"