Indeed all you have to do is signup as a contributor... However you need to contribute positively and it will be reviewed... Chances are if you are caught with self promo then I am sure you will be banned...
As Danny83 points out above its about educational/relevant content...
Also keep in mind Wikipedia links are NOFOLLOW... You may get some traffic.... and most search engines will not transfer any link juice wrt ranking on the SERPs - G for sure being one of them - the only exception I am aware of is duckduckgo (there may be others) whom have stated that they look at such authority sites and ignore NOFOLLOW in that specific case, that said duckduckgo has a very tiny market share and is nothing compared to the Giant G ;-) ...
Bottom line - I wouldn't bother trying to get a link on Wikipedia...