contrib/MSVC6.txt: Add guide how to use nasm in MSVC6

It's possible to use nasm as asm compiled right within
MSVC6 IDE. Lets point it out via a small guide.

Signed-off-by: Alexander Ilyin <dragity@mail.ru>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

contrib/MSVC6.txt

@@ -0,0 +1,25 @@
+Compilation with Nasm on MSVC 6.0, with usage of Custom Build step.
+
+1) Open your project in MSVC.
+
+2) Add .asm at the list of source files.
+
+3) Click on it with the right button, further press Settings...
+
+4) In General tab, flags "Always use custom build step" and "Exclude file
+   from build" should be disabled.
+
+5) In "Custom Build" tab it is necessary to define "Commands" and "Outputs"
+   (By "Outputs" build system checks if it has reached a necessary result.
+    This parameter is mandatory and without it MSVC will not let you close
+    the window by OK button).
+
+   The "Commands" should be set to
+
+        nasmw.exe -fwin -o $(OUTDIR)\$(InputName).obj $(InputName).asm
+
+   And "Outputs" to
+
+        $(OUTDIR)\$(InputName).obj
+
+Have fun!