Document 'ARM' build option for cmake_generic.sh.

Docs/GettingStarted.dox

@@ -100,6 +100,7 @@ A number of build options can be defined when invoking the build scripts or when
 
 |Build Option         |Value|Description          |
 |---------------------|-----|---------------------|
+|ARM                  |0|Configure project using ARM cross-compiler toolchain (cmake_generic.sh only), cmake-gui users need to specify ARM toolchain file for cross-compiling explicitly|
 |RPI                  |0|Configure project using Raspberry Pi cross-compiler toolchain (cmake_generic.sh only), cmake-gui users need to specify RPI toolchain file for cross-compiling explicitly|
 |MINGW                |0|Configure project using MinGW (32-bit or 64-bit) cross-compiler toolchain (cmake_generic.bat and cmake_generic.sh only), cmake-gui users need to specify MinGW toolchain file for cross-compiling explicitly|
 |WEB                  |0|Configure project using Emscripten cross-compiler toolchain (cmake_generic.bat and cmake_generic.sh only), cmake-gui users need to specify Emscripten toolchain file for cross-compiling explicitly|