doc: explicit about VS 2015 support in readme

`vcbuild.bat` already supports both Visual Studio 2013 and 2015.

PR-URL: https://github.com/nodejs/node/pull/5406
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: Rod Vagg <rod@vagg.org>

README.md

@@ -160,8 +160,8 @@ $ node -e "console.log('Hello from Node.js ' + process.version)"
 Prerequisites:
 
 * [Python 2.6 or 2.7](https://www.python.org/downloads/)
-* Visual Studio 2013 for Windows Desktop, or
+* Visual Studio 2013 / 2015, all editions including the Community edition, or
-* Visual Studio Express 2013 for Windows Desktop
+* Visual Studio Express 2013 / 2015 for Desktop
 * Basic Unix tools required for some tests,
   [Git for Windows](http://git-scm.com/download/win) includes Git Bash
   and tools which can be included in the global `PATH`.