[SC64][HW] Added README.md for shell design files

hw/shell/README.md

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+# SummerCart64 shell design files
+
+This folder contains several different designs of the SummerCart64 shell and button
+
+`./` folder contains original design made by [Mateusz Faderewski 'Polprzewodnikowy'](https://github.com/Polprzewodnikowy) in the Autodesk Fusion 360 and exported `.stl` files ready for 3D print.
+
+`./injection mold` folder contains adapted design for injection molding process by [Mena 'Pheeeeenom'](https://github.com/Pheeeeenom).
+
+`./button` folder contains designs for the button on the back of the SummerCart64. Designs are:
+- `sc64_shell_button_b3fs-101x.stl` - for the Omron B3FS-101x button (by [Mason Stooksbury](https://github.com/MasonStooksbury))
+- `sc64_shell_button_b3fs-105x.step` - for the Omron B3FS-105x button (by [Mena 'Pheeeeenom'](https://github.com/Pheeeeenom))
+- `sc64_shell_button_b3fs-105x_with_end_stop.step` - for the Omron B3FS-105x button but with additional flange to prevent it from coming out of the shell (by [Mena 'Pheeeeenom'](https://github.com/Pheeeeenom))