Achievements are something player can gain in game by reaching
set goals. Achievements active in the ruleset are defined in
game.ruleset.

Depending on whether achievement is defined unique or not,
they are granted only to first player, or all players,
to reach the goal for the achievement. Goal is defined by
achievement type and another value specific to that achievement type.


Achievement types
=================

Map_Known
    Achievement is granted when player has mapped at least
    <value>% of the world.

Spaceship
    This achievement is granted when player launches spaceship.
    <Value> is ignored.

Multicultural
    Achievement is granted when player has citizens of at least
    <value> different nationalities (across all their cities).

Cultured_City
    Achievement is granted when player has a city with at least
    <value> culture points.

Cultured_Nation
    Achievement is granted when player has at least <value>
    culture points.

Lucky
    Achievement is granted on turn when random number generator
    gives value less than <value> out of 10000.

Huts
    Achievement is granted once player has entered <value> huts.

Metropolis
    Achievement is granted once there's city of at least size
    <value> in the player's empire.

Literate
    Achievement is granted when player's literacy percent is at least
    <value>.

Land_Ahoy
    Achievement is granted when player has seen <value> different
    islands/continents. Home continent counts, so to give achievements
    for finding first other continent, use value 2.

Killer
    Achievement is granted when player has caused death of
    <value> enemy units.

Rich
    Achievement is granted when player's gold reserves are at least
    <value> gold.
