port r14072 from 1.1: Document a bit the mirage architecture

tools/perfmodels/README

@@ -0,0 +1,5 @@
+This contains performance models for some architectures and examples
+
+mirage is composed of:
+- 2 Intel Xeon X5650 @2.67GHz, thus 12 CPU cores
+- 3 NVidia GF100 Tesla M2070, thus 3 GPUs