jira-projects/HLS#2: HLS-11: Document Environment Variables

Previously HLS-11

There are now quite a few variables used within the script, many of which can be set prior to running it in order to avoid needing to pass command line options (allowing use of wrapper scripts).

It's probably worth creating a Markdown file containing a list of these and what they do/default to for easy reference (it'll also ensure that there aren't any which can only be set using an option).