Fixed and improved Getting Started documentation to be more failproof.

doc/getting-started.dox

@@ -53,7 +53,7 @@ polluted.
 
 @section getting-started-review Review project structure
 
-The base project consists of just seven files in two subfolders. %Magnum uses
+The base project consists of just six files in two subfolders. %Magnum uses
 CMake build system, see @ref cmake for more information.
 
     modules/FindCorrade.cmake
@@ -154,6 +154,9 @@ just build and run the application. If CMake isn't able to find the
 dependencies or the building fails for some reason, you might want to look at
 @ref building-windows-troubleshooting.
 
+If CMake complains about `GlutApplication` missing, you forgot to enable
+`WITH_GLUTAPPLICATION` when building %Magnum, @ref getting-started-download "go back and fix it".
+
 @image html getting-started.png
 @image latex getting-started.png