In fact vim-snippets is a sideplugin that provides general snippets for various programming languages thorugh plugins (like: UltiSnips, snipmate, ... ) that provide snippet support to vim.
so you should first have Ultisnips and vim-snippets plugins installed at the same time. then to add your desired snippet add the following code to vim-snippets/UltiSnips/markdown.snippets
file.
########################
# My Personal Snippets #
########################
snippet // "Comment"
<!-- ${1} -->
endsnippet
Of course vim-snippets is a general system and has some policis for writing snippets and you may after a while want to update this plugin and your custom snippets ( your personal ones ) will be gone. so its wiser to define your personal snippets in your dot files. In this case just make a file named markdown.snippets
in .vim/UltiSnips
folder and add the same lines of code to that file. This way your vim-snippets plugin is intact and also you have your personal snippets managed.