Updates installation instructions to support ARM64 (#5166)

1. If you are running Mac OS X, a pkg file (which looks similar to Anaconda3-5.3.0-MacOSX-x86\_64.pkg) will be downloaded. Double-click on the file to start the installation process.

1. After the installation has finished, open up a bash prompt in your terminal window.

1. If you have an ARM64 Mac (M1/M2) running Catalina (or later), additional prerequisites must be installed for ISIS to run in emulation:

 - Install [XQuartz](https://www.xquartz.org/). (Tested with XQuartz 2.8.5 on MacOS Catalina)

 - Install Rosetta2. From the terminal run: `/usr/sbin/softwareupdate --install-rosetta --agree-to-license`

 - Include the `# MacOS ARM64 Only` lines below

1. Next setup your Anaconda environment for ISIS. In the bash prompt, run the following commands:

    ```bash

    #MacOS ARM64 Only - Setup the new environment as an x86_64 environment

    export CONDA_SUBDIR=osx-64

    #Create a new conda environment to install ISIS in

    conda create -n isis

    #Activate the environment

    conda activate isis

    #MacOS ARM64 Only - Force installation of x86_64 packages instead of ARM64

    conda config --env --set subdir osx-64

    #Add the following channels to the environment

    conda config --env --add channels conda-forge

    conda config --env --add channels usgs-astrogeology