Expand "Configuring node images" to outline how to set up compute nodes for compilation

[jcwomack]

Since it is recommended to compile applicaitons on a compute node (rather than management node) I think it might be helpful to give a couple of general pointers to how to edit /home/citc/compute_image_extra.sh to get the node images ready for general compilation, e.g. uncomment

sudo yum -y groupinstall "Development Tools"

and install Python with unversioned Python

sudo yum -y install python3
sudo alternatives --set python /usr/bin/python3