There is no Markdown support for wikis and no addon exists that enhances the wiki with Markdown support. The one addon linked by Jose Angel Gonzalez Calvo is only for the document details / regular document content support. I have been experimenting with using the XWiki rendering engine to provide various alternative wiki markups in Alfresco (i.e. Confluence syntax and support for macros), and it was generally a promising approach, but since wikis are not often used in Alfresco and my (paid) projects required more attention, I haven't progressed that much. Since Alfresco is not intending for Share to be enhanced in any major way (and are even stripping away functionality), I doubt it would make sense to invest too much effort into Mardown support for wikis anyway....
