Markdown is a superset of HTML, inheriting its multiline comment syntax. HTML doesn't have a single-line comment, and nether does any variant of Markdown (that I know of), but JavaScript -- which also uses HTML's multiline comment syntax -- does.
I propose that it would fit both the spirit of the ecosystem and the function of the language to add JavaScript's // as a single-line comment syntax, and would be a meaningful quality of life addition to users.
Additionally, the other popular choice for this across other languages, #, isn't an option due to its existing use for headers.
(Up front, I do realize that it would require parsers to look first for whitespace, or really anything other than URL schemes, but while I recognize the tradeoff I at least believe that it's worth consideration.)