perf top/report: Add tips about a list option

Add two tips that describe --list option of config sub-command and
explain how to choose particular config file location.
Signed-off-by: Nambong Ha <over3025@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Taeung Song <taeung@kosslab.kr>
Link: http://lkml.kernel.org/r/1475191562-3240-1-git-send-email-over3025@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/tips.txt

@@ -30,3 +30,5 @@ If you prefer Intel style assembly, try: perf annotate -M intel
 For hierarchical output, try: perf report --hierarchy
 Order by the overhead of source file name and line number: perf report -s srcline
 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