All wiki entries that are referenced from within the game when you right-click on an Astrology/Item/Creature/Skill/Characteristic/Player/Religion/Structure/Terrain and select "What´s This".
The client has limited support for Wiki markup (it only supports the <nowiki><br></nowiki> HTML tag) currently so just use plain text in these pages.
If you make an article, please add <nowiki>[[Category:Ingame Help]]</nowiki> to it. It will automagically show up here.
It´s probably easiest if, at least to begin with, the Player:<player name> entries are redirected to User:<player name>.