GitLab

Some gentle reformatting to make the Markdown more standards-compliant, so that it renders correctly on Github (still renders correctly on Gitlab as well).

Here's how the old version looks like on Github: https://github.com/agraef/purr-data/tree/master (note that the line breaks and the formatting of the verbatim code sections is badly garbled)

The new version looks all right: https://github.com/agraef/purr-data/tree/readme-fixes

The new version also looks all right on Gitlab: https://git.purrdata.net/aggraef/purr-data/tree/readme-fixes