Add PerpleX output-maker use-comments

[bobmyhill]

create_burnman_readable_perplex_table.py needs a few more details for users:

• Make it clear that the PerpleX output script requires the full path and executable name for werami, and the project name without .dat.
• Make a note of which files need to be moved to the current directory/which would work with global path names

[bobmyhill]

Redundant. Full workflows provided by #641 and #642.