Document VS 2013 as minimum VS version

This is part of removing support for VS versions older than VS 2013.
See #2465 for details and feedback.

Related to #2465

README.md

@@ -79,6 +79,8 @@ more information.
 
 ## System requirements
 
+GLFW is written in C99 and does not support Visual Studio 2012 or earlier.
+
 GLFW supports Windows XP and later and macOS 10.8 and later.  Linux and other
 Unix-like systems running the X Window System are supported even without
 a desktop environment or modern extensions, although some features require

docs/compile.dox

@@ -7,6 +7,8 @@
 This is about compiling the GLFW library itself.  For information on how to
 build applications that use GLFW, see @ref build_guide.
 
+GLFW uses some C99 features and does not support Visual Studio 2012 and earlier.
+
 
 @section compile_cmake Using CMake