added message that timings doesn't include data transfer to gpu performance sample (Issue #1846)

samples/gpu/performance/performance.cpp

@@ -217,6 +217,8 @@ int main(int argc, const char* argv[])
 
     TestSystem::instance().setNumIters(iters);
 
+    cout << "\nNote: the timings for GPU don't include data transfer" << endl;
+
     TestSystem::instance().run();
 
     return 0;