Add explanation how to use makefile.vc

makefile.vc

diff --git a/makefile.vc b/makefile.vc
index 1cacd2f..fecc0ee 100644
--- a/makefile.vc
+++ b/makefile.vc
@@ -1,6 +1,12 @@
 # Makefile for Visual Studio

 # Written by Krzysztof Kowalczyk (http://blog.kowalczyk.info)

 # This code is in public domain.

+# Use it like this from command line:

+# * add Visual Studio tools to your %PATH%

+# * nmake -f makefile.vc TARGET=rel

+#   or

+# * nmake -f makefile.vc TARGET=dbg

+# * the result is perftest.exe in either rel or dbg directory

 CC = cl.exe

 LD = link.exe