Add some API info

API.txt

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+# API
+
+This document is a WORK IN PROGRESS.
+
+Currently, it only contains some information about the used groups.
+
+
+Groups
+
+	dig_by_water=1
+		Blocks with this group will drop when they are near flowing water
+
+
+Groups (mostly) used for crafting recipes
+
+	sand=1
+		Sand (any color)
+
+	sandstone=1
+		(Yellow) sandstone and related nodes (chiseled and the like) (only full blocks)
+
+	redsandstone=1
+		Red sandstone and related nodes (chiseled and the like) (only full blocks)
+
+	quartz_block=1
+		Quartz Block and variants (chiseled, pillar, etc.) (only full blocks)
+
+	tree=1
+		Oak Wood, Birch Wood, etc. (tree trunks)
+
+	wood=1
+		Oak Wood Planks, Birch Wood Planks, etc. (only full blocks)
+
+	wood_slab=1
+		Slabs made out of wooden planks
+
+	coal=1
+		Coal of any kind (coal lump)
+
+
+Other groups
+	water=1
+		Water
+
+	lava=1
+		Lava