added explicit compiler flag to win32 build notes

svn path=/trunk/matplotlib/; revision=4859

Author: cmoad

setupext.py

@@ -37,7 +37,7 @@
   http://matplotlib.sourceforge.net/win32_static_vs.tar.gz and
   see the README in that dir
 
-  > python setup.py build bdist_wininst
+  > python setup.py build --compiler=msvc bdist_wininst
 
 """