READMEs aren't necessarily markdown. That is the most popular option, but there's others out there. Here's Github's list of supported README formats for example:

The following markups are supported. The dependencies listed are required if you wish to run the library. You can also run script/bootstrap to fetch them all.

• .markdown, .mdown, .mkdn, .md -- gem install commonmarker (https://github.com/gjtorikian/commonmarker)
• .textile -- gem install RedCloth (https://github.com/jgarber/redcloth)
• .rdoc -- gem install rdoc -v 3.6.1
• .org -- gem install org-ruby (https://github.com/wallyqs/org-ruby)
• .creole -- gem install creole (https://github.com/larsch/creole)
• .mediawiki, .wiki -- gem install wikicloth (https://github.com/nricciar/wikicloth)
• .rst -- pip install docutils
• .asciidoc, .adoc, .asc -- gem install asciidoctor (http://asciidoctor.org)
• .pod -- Pod::Simple::XHTML comes with Perl >= 5.10. Lower versions should install Pod::Simple from CPAN.