Add file explaining registers

register_notes

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+$2100 - screen on/off + brightness
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+$2105 - screen format register, changes size of tiles
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+$2121 - stores the index into CGRAM to write a color value via $2122
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+$2122 - write a 2-byte SNES color value to the CGRAM. this auto-increments after writing,
+which is why $2121 needs to be written to reset this. it's common to write `stz $2121`
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