no markdown libraries

[DouweKoopmans]

there should be a category for markdown rendering libraries.

to list a few examples:
pegdown
commonmark-java
txtmark
markdown4j
markdownpapers

[jbduncan]

If a Markdown category were to be considered, I think it should really be a general category for Text-to-text Converters, which would include not only Markdown libs but also Asciidoc lib(s) like AsciidoctorJ.

[akullpp]

Please create a pull request with the most awesome and active markdown libraries. However, you should consider to add a more general category like @jbduncan said.