No GH syntax highlighting for README.md

[eklitzke]

Pandoc is generating markdown that differs from GitHub-flavored markdown. In particular, it's not using the correct GH-flavored markdown for syntax highlighting.

Interestingly, just deleting the README.md file (and retaining the README.rst file) fixes this issue, see https://github.com/eklitzke/okcupyd/tree/syntax

Pandoc also supports GH-flavored markdown, using the type markdown_github instead of markdown; but just changing that in the pre-commit hook breaks the .rst generation.