README: upgrade = extract-over-top (preserves bindings.txt + volume.cfg)

The advertised upgrade flow was delete-and-unzip, which wipes the two
per-player files (key/joystick bindings incl. joyconfig output, saved
volume) every version.  The zip carries neither file, so extract-over-
top preserves them by construction -- now the documented flow, with the
copy-out fallback for clean-delete preferrers.

players/README.txt

@@ -5,6 +5,14 @@ IMPORTANT: FULLY EXTRACT this zip first (right-click - Extract All).
 Do NOT run anything from inside the zip.  The .bat files must sit next
 to the 'content' and 'build' folders.
 
+UPGRADING from an older version?  Extract the new zip OVER your
+existing game folder and say YES TO OVERWRITE.  Do NOT delete the
+folder first -- your personal files (contentindings.txt with your
+key/joystick setup, contentolume.cfg with your volume) are not in
+the zip and survive an extract-over-top, but a delete wipes them.
+If you prefer a clean delete anyway, copy those two files out first
+and put them back after.
+
 TO JOIN AN INTERNET GAME:   double-click  join.bat
    -- pick your CALLSIGN and MECH, click JOIN (your choice is sent to
    the operator; waits for the session if it's not up yet)