Docs: Clarify what can be output

[guyfawcus]

There is a bit of inconsistency in the docs as to what archmap can generate.
With 6dc13e0 CSV files are now output by default and if #15 goes ahead, some housekeeping is going to have to be done.

This issue is to track those changes and to see if anyone has any suggestions for things like the description which is currently GeoJSON/KML generator

• Change description
• Mention more about CSV output in docs
• Mention more about text output in docs
• Rename "archmap_users.txt" -> "archmap.txt" in docs (if Break backwards compatibility? (users -> text) #15)