doc: mention GLSL BOM issues in Windows troubleshooting guide.

Even though any editor on any platform can add BOM, no serious ones do
that by default. Thus this is pretty localized to Windows where Notepad
adds BOM implicitly.

doc/platforms-windows.dox

@@ -268,6 +268,21 @@ set_property(DIRECTORY ${PROJECT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT MyAppli
 
 The Magnum bootstrap projects set this automatically.
 
+@subsection platform-windows-troubleshooting-bom-in-shaders UTF-8 BOM in shader code
+
+If your GLSL shaders fail to compile with the following error, it means you
+have your file saved as UTF-8 with a BOM:
+
+@code{.shell-session}
+GL::Shader::compile(): compilation of vertex shader 1 failed with the following message:
+ERROR:2:1: `` : syntax error: illegal extended ASCII character (0xef)
+ERROR:2:1: `` : syntax error: illegal extended ASCII character (0xbb)
+ERROR:2:1: `` : syntax error: illegal extended ASCII character (0xbf)
+@endcode
+
+While the C++ compilers can usually ignore the BOM marker, it's not so common
+with GLSL compilers in the drivers. Save your file as UTF-8 without BOM.
+
 @todoc DLL paths
 @todoc vcpkg
 @todoc desktop ES