* lisp/ox-md.el (org-md-headline-style): New setting `mixed' allows
for mixing Setext and ATX-style headlines when exporting to Markdown.
* doc/org-manual.org (Header and sectioning structure): Document
`mixed' headline style.
* etc/ORG-NEWS (New allowed value of ~org-md-headline-style~ to mix
ATX and Setext style headlines): Document the new feature.
When exporting to Markdown, there was previously no obvious way of
mixing Setext-style and ATX-style headlines. Via the
`org-md-headline-style' custom setting, headlines could be exported as
either all-Setext or all-ATX, but not as a mix.
With this change setting `org-md-headline-style' to `'mixed' will
export the first two headline levels as Setext-style Markdown
headlines and the rest as ATX-style.
TINYCHANGE
1a7d7a5a5b