perf_counter tools: Add CREDITS file for Git contributors

Much of perf's libraries comes from the Git project. I noticed
that the files (in tools/perf/util/*.[ch] and elsewhere) are
quite spartan wrt. credits, so lets add a CREDITS file that
includes an (incomplete!) list of main contributors.

Thanks guys, these libraries are really useful. Special thanks
go to Johannes Schindelin and Junio C Hamano for coming up with
this list.

List-Composed-By: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>