Don't assume release notes are markdown?

Currently we'll detect and slurp up RELEASES.rst but then embed the contents in markdown. Oops!

I don't know if Github Releases supports not-markdown, or if we actually need to convert the user's notes to markdown.

Inspect the code path that detects and embeds RELEASES.rst, then verify whether GitHub Releases accepts reStructuredText or requires Markdown. Confirm the expected behavior for non-Markdown release notes and make the handling consistent with that requirement.

Written by the indexing model from the issue text.