perf_counter: create Documentation/perf_counter/ and move perfcounters.txt there

We'll have more files in that directory, prepare for that.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/Documentation/perf-counters.txt b/Documentation/perf_counter/design.txt
similarity index 100%
rename from Documentation/perf-counters.txt
rename to Documentation/perf_counter/design.txt