add scribbltests howto

src/howto/scribbltests.md

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+# Scribbltests
+
+[Scribbltests](https://github.com/Hacktix/scribbltests)
+are actually compiled with [RGBDS](https://github.com/gbdev/rgbds).
+However, as the binaries are already part of the repository,
+we just use them.
+
+## Game Boy Devices & SoC Revisions
+
+All test roms have been verified on
+[MGB 9638 D](https://gbhwdb.gekkio.fi/consoles/mgb) and
+[CPU CGB D](https://gbhwdb.gekkio.fi/consoles/cgb) by their author.
+
+## Exit Condition
+
+Running for around 10 frames should be enough for most tests.
+The only exception is `statcount_auto` which should be run for around 270 frames
+or 4.5 seconds (emulated time).
+
+## Test Success/Failure
+
+Test success/failure can be determined by
+[screenshot](https://github.com/c-sp/gameboy-test-roms/tree/master/src/scribbltests/)
+comparison.
+Note that there are no screenshots for `failrylake` and `winpos` at the moment.
+
+To create comparable screenshots,
+your emulator should calculate colors as follows:
+
+- `#000000`, `#555555`, `#AAAAAA` and `#FFFFFF` are used for the four DMG LCD
+  shades
+- LCD shades for CGB compatibility mode are:
+  - background: `#000000`, `#0063C6`, `#7BFF31` and `#FFFFFF`
+  - objects: `#000000`, `#943939`, `#FF8484` and `#FFFFFF`