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>

tools/perf/CREDITS

+Most of the infrastructure that 'perf' uses here has been reused
+from the Git project, as of version:
+
+    66996ec: Sync with 1.6.2.4
+
+Here is an (incomplete!) list of main contributors to those files
+in util/* and elsewhere:
+
+ Alex Riesen
+ Christian Couder
+ Dmitry Potapov
+ Jeff King
+ Johannes Schindelin
+ Johannes Sixt
+ Junio C Hamano
+ Linus Torvalds
+ Matthias Kestenholz
+ Michal Ostrowski
+ Miklos Vajna
+ Petr Baudis
+ Pierre Habouzit
+ René Scharfe
+ Samuel Tardieu
+ Shawn O. Pearce
+ Steffen Prohaska
+ Steve Haslam
+
+Thanks guys!
+
+The full history of the files can be found in the upstream Git commits.