add api document

API.md

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+
+```lua
+-- register a new nutrient
+balanced_diet.register_nutrient(name, {
+    description = S(name),
+    on_eat = function(player, value)
+        -- optional. called once when a food item is eaten.
+    end,
+    apply_value = function(player, value)
+        -- how to apply the nutrient value to the player. the nutrient value is a function of what the player has
+        -- eaten, how much of the nutrient individual foods provide, and how long ago they ate the item.
+    end,
+})
+
+-- override a nutrient
+balanced_diet.override_nutrient(name, def)  -- as above
+```
+
+* `balanced_diet.is_food(item_or_stack)`
+
+  returns true if the item is a registered food.
+
+```lua
+-- register an *existing item* as a food
+balanced_diet.register_food("default:apple", {
+	-- item_eat(2)
+	category = "apple",  -- other apples cannot be eaten at the same time.
+	replace_with = "default:stick",
+	saturation = 1,  -- adds this amount to the player's saturation
+	duration = 300,  -- how long the food lasts.
+	nutrients = {
+		carbohydrate = 1,
+		vitamin = 1,
+	},
+	after_eat = function(itemstack, eater, pointed_thing)
+		-- called after an item is eaten, and can trigger food-specific effects
+        -- return true to prevent taking an item from the stack.
+	end,
+})
+
+-- override a food
+balanced_diet.override_food(item_name, overrides)
+```
+
+```lua
+balanced_diet.register_on_item_eat(function(eater, itemstack, pointed_thing)
+    -- if this returns true, this will block the item from being eaten via the normal mechanic
+end)
+```
+
+```lua
+balanced_diet.register_after_item_eat(function(eater, itemstack, pointed_thing)
+    -- called *after* a food is eaten. return true to prevent taking an item from the stack.
+end)
+
+```