Added readme and screenshot

README.md

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+# 256 Dyes for Minetest
+256 variants of a single item "256_dyes:dye". Not useful for now.
+Universal mixing craft recipe: you can mix up to 9 colors at once on the crafting grid.
+In game, only the brown dye can be found in creative inventory. You can change the color by clicking:
+- Right click changes hue
+- Left click changes brightness
+The `color.dat` file is used by the mixing algorithm. (compressed list of RGB color codes)
+gen_palette.py: Python 3 script to run with Python Image Library to generate the palette and color.dat. Could be easily modified to generate a different palette.
+
+256-colors palette based on 24 hues, with 10 variations of each one, and grey scales.
+
+Screenshot showing the 24 pure hues:
+![Screenshot](https://raw.githubusercontent.com/Gael-de-Sailly/256_dyes/master/screenshot.png)
+
+Code: Gael-de-Sailly
+Textures:
+- palettes: Gael-de-Sailly
+- white dye texture: Celeron55, modified by Gael-de-Sailly