[kfox]

Adds docs/GEOS64_Usage.md, a user guide for the GEOS 64 menu item, and links it from the README's usage documents.

It covers:

• What GEOS 64 on the TeensyROM is, the boards it runs on (TR and TR+), the USB mouse, and where it is in the menu (Utilities).
• SD card setup: the /GEOS64 folder, D81 images only, and the boot disk it looks for (DESK TOP and GEOBOOT).
• The -TR copy: made on the first launch, reused afterwards, and how to start over from the original.
• GeoRAM: preload from SD:/georam.ram, saving with the Alt button on the TR+ (LED flash, georam.bak), and why GEOS does not set it up as a RAM disk.
• Date and time from the TR's clock and time zone.
• The messages shown when GEOS 64 can't start, and what each means.

Every statement was checked against the code on main (GeosLauncher.ino, GEOS/, IOH_GeoRAM.c, RAMBanks.c, MainMenuItems.h). Documentation only; no code changes.

[SensoriumEmbedded]

Looks great, thank you much!