Updates installation instructions to support ARM64 (#5166)

Resolves DOI-USGS/ISIS3#5164

README.md

@@ -51,15 +51,26 @@ This installation guide is for ISIS users interested in installing ISIS (3.6.0)+
 
 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