Activate roxygen markdown mode in DESCRIPTION

[brookslogan]

We already have some markdown syntax in the roxygen comments, so activate markdown mode for this package.

[nmdefries]

Removed link formatting in f67235a to get tests to pass. Links didn't include URL, but roxygen doesn't seem to automatically create package links, only topic and package::function. #2 substantially changes data docs anyway, so we can change this.

[nmdefries]

Looks good.