Bukkit

The config.yml is created the first time you run the plugin.
Everytime the plugin is enabled, any worlds not in the config.yml will be added.
Any world not in the config.yml should then adopt the default settings.
Each world will have a block with it's name at the top.
Please leave the default in.
The default config.yml without any worlds added can be found here
Here is an explanation of every option:

• async: True enables the experimental async mode that lets the timers run in parallel and the spawn location validation keeps looking for a suitable spawn location until it finds one instead of giving up after 89 fails. Can cause errors if the timer is less than 5 seconds. False disables this experimental mode and instead enables the more stable sync mode.
• multi: True if each world uses it's own settings. False if all worlds should use default
• checks: Selection of 'high', 'low', and 'none'. This dictates how strict the spawn location validation is. 'none' doesn't run any checks, 'low' checks that the block is air and the one below it is not and 'high' checks that the block is air, the two above it are air and the one below it is not air.
• timer: Time is seconds between each Horde attack
• number of monsters: the number of monsters to be spawned. If 0, a random number will be selected
• random cap: if 'number of monsters' is 0, this is the maximum number of monsters that can be randomly selceted
• min and max distance: the minimum and maximum distance the creature spawns can be from the randomly targeted player
• sunburn: False to prevent monsters from burning in sunlight. True to let them suffer
• time start and stop: These are the two points of in-game time in which the spawns can occur. The times are like real time times 1000 and 0 is roughly morning(I think!). So spawns just at night would be 'time start: 18000' and 'time stop: 24000'
• server message: Message to be broadcast to the server after every Horde attack. Put '' to have no message sent. There are five placeholders that can be used:
  • $Player: the players (or player) have been targeted
  • $Creature: the creatures (or creature) have have been spawned
  • $World: the world the horde attack took place on
  • $p_was/were: puts 'was' if there is only one player. Puts 'were' if there was more than one
  • $c_was/were: puts 'was' if there is only one creature. Puts 'were' if there was more than one
• player message: Message to be sent to the targeted player on every spawn. Put '' to have no message sent. There are two placeholders that can be used:
  • $Player: the player that has been targeted
  • $Creature: the creature that has have been spawned
• creatures: Options for each creature type. For each true will cause that creature type to be available to be randomly spawned. False will mean it can't be spawned
  
  

Example
So say I wanted all my worlds to have the same settings, experimental async to be disabled, the checking to be high, the attacks to happen every minute, a random number of creatures with a maximum of 15 to spawn, the spawns to happen at a minimum distance of 10 blocks but no further than 30 blocks away, mobs not to burn in sunlight, the spawns to be able to happen at any in-game time, no messages to be sent and all creatures but monsters and ender dragons, the config file would look like this (which is not formatted correctly):

async: false
multi: false
checks: high
Default:
timer: 60
number of monsters: 0
random cap: 15
min distance: 10
max distance: 30
sunburn: false
time start: 0
time stop: 24000
server message: ''
player message: ''
creature:
blaze: true
cave spider: true
chicken: true
cow: true
creeper: true
ender dragon: false
enderman: true
ghast: true
giant: true
monster: false
mushroom cow: true
pig: true
pig zombie: true
sheep: true
silverfish: true
skeleton: true
slime: true
spider: true
squid: true
villager: true
wolf: true
zombie: true