📝 Fix Ender-3 V2 Bootscreen README (#993)

Author: thisiskeithb

config/examples/Creality/Ender-3 V2/LCD Files/Custom Bootscreens/Custom Boot Screen Instructions.md

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+# Making a Custom Bootscreen
+
+To make a custom bootscreen there are 5 requirements:
+
+1. The file format must be JPEG.
+2. Progressive must be tuned off in the export settings (under "Advanced" in GIMP).
+3. The Sub-sampling format must be chroma quartered (4:2:0) (also under "Advanced" in GIMP).
+4. The resolution must be 480x272.
+
+> [!NOTE]
+> The left side of the image will be the top. You can compose the image oriented upright as 272x480, but it must be rotated prior to export. (Rotating *during* export only applies a "rotation" attribute that won't be parsed by DWIN DACAI.)
+
+5. For DACAI the file must be named "`0`" and placed in `private/image`. For DWIN the file must be named "`0_start`" and placed in `DWIN_SET`.
+
+After replacing the Bootscreen follow the usual screen flashing instructions.