Currently Stash supports Markdown as markup language. It can be used in descriptions/comments and in README file. But other providers like Bitbucket, also offers the possibility to use AsciiDoc format as markup language in README file.
In my opinion providing AsciiDoc support will a really improvement/step forward in Stash and its documentation engine because of the richness offered by AsciiDoc language to cite a few options (tables, fashion inner elements, TOC, ...).
In case of AsciiDoc format, there are several accepted extensions like .ad, .adoc or .asciidoc, which differs from markdown where the extension is .md.