update cross compile toolchain installation instructions in README

[capsulecorplab]

The cross compile toolchain for Teensyduino seems to be expecting arduino-1.8.13 to be installed in /opt as well as arm-none-eabi-gcc to be installed in /opt/arduino-1.8.13/hardware/tools/arm/bin/.

For example, I'm currently running into the following error, after installing arduino in the expected folder.

CMake Error at CMakeLists.txt:17 (project):
  The CMAKE_C_COMPILER:

    /opt/arduino-1.8.13/hardware/tools/arm/bin/arm-none-eabi-gcc

  is not a full path to an existing compiler tool.

  Tell CMake where to find the compiler by setting either the environment
  variable "CC" or the CMake cache entry CMAKE_C_COMPILER to the full path to
  the compiler, or to the compiler name if it is in the PATH.


CMake Error at CMakeLists.txt:17 (project):
-- Configuring incomplete, errors occurred!
  The CMAKE_CXX_COMPILER:
See also "/home/kasm-user/fprime-arduino/build-fprime-automatic-Teensyduino/CMakeFiles/CMakeOutput.log".

See also "/home/kasm-user/fprime-arduino/build-fprime-automatic-Teensyduino/CMakeFiles/CMakeError.log".
    /opt/arduino-1.8.13/hardware/tools/arm/bin/arm-none-eabi-g++

  is not a full path to an existing compiler tool.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.


[ERROR] CMake erred with return code 1. Partial build cache remains. Run purge to clean-up.