perf buildid-cache: Document for Node.js USDT

Add a tip for Node.js USDT(User-Level Statically Defined Tracing) probes
in tips.txt
Signed-off-by: Hansuk Hong <flavono123@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20171123160546.9722-1-flavono123@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/tips.txt

@@ -32,3 +32,4 @@ Order by the overhead of source file name and line number: perf report -s srclin
 System-wide collection from all CPUs: perf record -a
 Show current config key-value pairs: perf config --list
 Show user configuration overrides: perf config --user --list
+To add Node.js USDT(User-Level Statically Defined Tracing): perf buildid-cache --add `which node`