--readme option fails to select markdown files other than README.md

[xuan-cao-swi]

Rubydoc.info seems always choose the README.md as the index: https://www.rubydoc.info/gems/hola-xuan

It's working on my local yardoc server (e.g. through yardoc; yard server)

Here is my .yardopts

--no-private
--markup markdown
--readme yardoc_frontpage.md
lib/hola.rb
lib/hello.rb

and the yardoc_frontpage.md

# Welcome to the Hola Xuan

This is a test gem

## Headline 2

Empty

### Headline 3

Empty

My testing gem location: https://github.com/xuan-cao-swi/sample-ruby-gem