Update build instructions with the VS info.

Author: nitrocaster

doc/howto/build.txt

@@ -2,7 +2,8 @@ TITLE: X-Ray 1.6 build and setup instructions
 
 Build:
 ------
-Currently, the only compiler supported is Visual C++ 2015.
+Currently, the only compiler supported is Visual C++ 2015. If you don't have Visual Studio yet, you can
+download and install Visual Studio 2015 Community Edition - it's absolutely free for open source projects.
 
 By default, all libraries mentioned below (except Windows/DirectX SDK) are assumed to be in
 the src\Externals directory.