> This contribution will be welcome.

I've attached a patch which implements the change. I followed the pattern that
ox-html uses to the greatest extent possible. I tested it by exporting org-mode
files to markdown with the table of contents both enabled and disabled. I
didn't see any errors, and I was able to make the top level heading a level 2
(or even level 3) heading rather than level 1.

Thanks,
Rohit