Add basic developer documentation

DEVELOPERS.md

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+# Developer information
+
+This file explains information to document the code of Repixture and information for people
+wanting to develop or mod Repixture.
+
+Note the focus of Repixture is not to be a "base for modding", but to be a standalone
+game in its own right. That having said, Repixture also doesn't actively oppose
+modding.
+
+## Some modding rules
+
+* Crafting recipes (except cooking and fuel recipes) **MUST**
+  be registered through `rp_crafting`.
+* Modifying player physics (like running speed)
+  **MUST** be done via `rp_player_effects`.
+
+## Mod APIs
+
+Some mods provide APIs to interact with. Check out the respective mod directories for a
+file named `API.md`.
+
+Mods with documented APIs:
+
+* `rp_crafting`: Add crafting recipes
+* `rp_achievements`: Add and trigger achievements
+* `rp_door`: Add doors
+* `rp_player_effects`: Add player effects (required if you want to modify player physics)
+* `tt`: Custom tooltips
+
+Beware: Calling functions that are not documented are NOT guaranted to be stable
+in future versions, so use them with care.
+
+## Groups
+
+A reference of all groups used in this game can be found in `DEV_GROUPS.md`.

DEV_GROUPS.md

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+# Group reference
+
+This file is developer documentation that documents all the groups used in this game.
+
+## Item groups
+
+## Interactive item groups
+* `not_in_creative_inventory`: Item won't show up in Creative Inventory
+* `not_in_craft_guide`: Item won't show up in crafting guide
+* `no_item_drop`: This item can't exist as a dropped item on the ground. When dropping it, it is deleted instantly
+
+### Tools
+* `axe`: Axe
+* `shears`: Shears
+* `shovel`: Shovel
+* `sword`: Sword
+* `spear`: Spear
+* `supertool`: Super tool, i.e. a powerful tool for Creative Mode use only
+* `sheep_cuts`: For shears. Rating specifies how often it can shear sheep
+
+## Armor
+* `is_armor`: Item is an armor piece
+* `armor`: Item is an armor piece and rating specifies armor percentage
+* `armor_material`: Rating specifies armor material; number is assigned dynamically. Equal number means equal material.
+* `armor_slot`: Which slots the armor piece belongs to. 1 = Helmet, 2 = Chestplate, 3 = Boots
+
+### Other item categorizations
+* `stick`: Stick
+* `bucket`: Any bucket
+* `bucket_water`: Bucket with water
+* `food`: Can be eaten. 2 = eatable. 3 = drinkable
+* `nav_compass`: Is a compass. Rating = current pointing direction (1 to 8)
+* `spawn_egg`: Item that spawns mobs
+
+
+
+## Node groups
+
+### Digging groups
+* `choppy`: Can be dug by brute force, like wood
+* `cracky`: Hard material like stone
+* `crumbly`: Soft material like dirt
+* `snappy`: Can be dug with fine tools like shears
+* `fleshy`: Node represents some kind of (semi-)living organism
+* `handy`: Can be dug with bare hand
+* `oddly_breakable_by_hand`: Can be dug with bare hand
+
+### Interactive node groups:
+* `soil`: For blocks that allow several plants to grow
+* `leafdecay`: Node decays if not close to a `tree` group node (max. distance = rating).
+               Decaying will destroy the node and release the item drop except dropping itself.
+* `leafdecay_drop`: Must be used in combination with `leafdecay`.
+                    Decaying will release the item drop, and it can even drop itself.
+* `magnetic`: Node is magnetic and can magnetize stuff (like compass)
+* `unmagnetic`: Node is "unmagnetic", this means it can de-magnetize stuff
+* `locked`: Node is considered to be locked
+
+### Node categorization
+
+* `dirt`: Any dirt (with or without cover)
+* `normal_dirt`: Any "normal" common dirt found in most temperate biomes (i.e. not swamp dirt or dry dirt) (with or without cover)
+* `dry_dirt`: Any dry dirt (with or without cover)
+* `swamp_dirt`: Any swamp dirt (with or without cover)
+* `grass_cover`: This node is covered with grass
+
+* `grass`: Any grass clump
+* `green_grass`: Any grass clump that is lush (“green”)
+* `normal_grass`: "Normal" grass clump
+* `dry_grass`: Dry grass clump
+* `swamp_grass`: Swamp grass clump
+
+* `plant`: Any node that is a plant
+* `farming_plant`: Any plant used for farming
+* `plant_cotton`: Cotton plant (rating = growth stage)
+* `plant_wheat`: Wheat plant (rating = growth stage)
+* `sapling`: Sapling
+* `fern`: Fern
+* `flower`: Flower
+* `seed`: Seed
+
+* `leaves`: Any leaves
+* `dry_leaves`: Dry leaves
+* `lush_leaves`: Any non-dry leaves
+
+* `plantable_dry`: You can plant farming plants on it and this node is considered to be dry
+* `plantable_sandy`: You can plant farming plants on it and this node is considered to be made out of sand
+* `plantable_soil`: You can plant farming plants on it and this node is considered to be "normal" soil
+* `plantable_fertilizer`: This node is fertilized
+
+#### Shaped
+
+* `slab`: Slab
+* `stair`: Stair
+* `path`: A path node like the Dirt Path
+* `door`: Any door
+* `door_wood`: Wooden door
+* `fence`: Fence
+* `sign`: Sign
+* `bed`: Bed
+* `torch`: Torch
+
+### Node material groups (used for full nodes only):
+
+* `planks`: Wooden planks
+* `wood`: Made out of wood
+* `tree`: Tree trunks
+* `stone`: Stone
+* `sand`: Sand
+* `sandstone`: Sandstone
+* `glass`: Glass
+* `fuzzy`: Wool, cotton bale, etc.
+
+### Liquids
+* `liquid`: Any liquid
+* `water`: Any water
+* `flowing_water`: Flowing water
+* `river_water`: Any river water
+* `swamp_water`: Any swamp water
+
+### Unused node groups
+
+* `flammable`
+
+
+
+## Damage groups
+
+* `fleshy`: The damage group used for everything that takes damage