This patch addresses minor issues in the markdown files, that can create
rendering problems, and also makes some minor improvements.
- Lists and code blocks should always be surrounded by blank lines
- Removed indentation of the top-level list in CONTRIBUTING.md,
that made it impossible for cmark, the CommonMark reference
implementation, to render the nested list.
- Surrounded bare URLs and email addresses with angled brackets; this way
bare URLs automatically gain an HTML <a href> tag, and email addresses
do not explicitly need to be linked to a mailto: URI.
