Document how to enable hunter spawns

[maxanier]

data/vampirism/forge/biome_modifier/spawn/hunter_spawns.json ?

{
  "type": "forge:add_spawns",
  "biomes": "#vampirism:has_spawn/hunter",
  "spawners": {
    "type": "vampirism:hunter",
    "maxCount": 3,
    "minCount": 1,
    "weight": 30
  }
}