Add Visual Studio 2010 SP1 download URL

Many people are confused by indicating just Visual Studio 2010 Express without SP1

docs/build-instructions/windows.md

@@ -3,7 +3,7 @@
 ## Requirements
 
   * Windows 7 or later
-  * [Visual C++ 2010 SP1 Express](http://www.visualstudio.com/en-us/downloads/download-visual-studio-vs#DownloadFamilies_4)
+  * [Visual C++ 2010 Express](http://www.visualstudio.com/en-us/downloads/download-visual-studio-vs#DownloadFamilies_4) and [SP1](http://www.microsoft.com/en-us/download/details.aspx?id=23691)
   * [node.js - 32bit](http://nodejs.org/download/) v0.10.x
   * [Python 2.7.x](http://www.python.org/download/)
   * [GitHub for Windows](http://windows.github.com/)