Add a note on using the version of SDL2 provided by the package manager instead of building from externals on Debian.

Building-for-Linux.md

@@ -46,6 +46,7 @@ Dependencies are listed here as commands that can be copied/pasted. Of course, t
   - Ubuntu 20.04, Linux Mint 20, or Debian Bullseye or later is required.
   -  Users need to manually specify building with QT Web Engine enabled.  This is done using the parameter `-DYUZU_USE_QT_WEB_ENGINE=ON` when running CMake. 
   - Users need to manually specify building with GCC 10. This can be done by adding the parameters `-DCMAKE_C_COMPILER=gcc-10 -DCMAKE_CXX_COMPILER=g++-10` when running CMake. i.e.
+  - Users need to manually disable building SDL2 from externals if they intend to use the version provided by their system by adding the parameters `-DYUZU_USE_EXTERNAL_SDL2=OFF`
 
 ```
 cmake .. -GNinja -DCMAKE_C_COMPILER=gcc-10 -DCMAKE_CXX_COMPILER=g++-10