Moved to xtrn/minesweeper

Created sysop documentation (.txt file)

If you were a lucky "early adopter" and had rev 1.7 running on your BBS

you probably noticed the following important changes introduced in rev 1.8:
* exec/minesweeper.js was moved to xtrn/minesweeper/minesweeper.js
* text/minesweeper.hlp was moved to xtrn/minesweeper/minesweeper.hlp

* data/minesweeper.json was moved to xtrn/minesweeper/winners.jsonl



If you already had game winners and want to retain them in the list, copy
or rename the data/minesweeper.jsonl to xtrn/minesweeper/winners.jsonl.
If you don't care, the game will run fine and new winners will be added to
the new/correct filename.