There really isn’t an accepted standard for comments in Markdown, and I’m not planning to add something special for Deepdwn.
This is really a markdown issue, I’m afraid, but you do have a few options, and it depends on what your intended use is.
The most common option is probably:
[comment]: # (your comment goes here)
This uses a link label that isn’t referenced in your document, and so doesn’t appear in the output. There are a few different formats for this, but the above is probably the best version.
Another option is to use HTML comments:
<!-- comment here -->
These are actually stripped from the output entirely in Deepdwn, but they’ll likely appear in your HTML source if you’re using Deepdwn to manage your blog posts (or similar) where some other tool is responsible for the rendering the HTML.
Some engines also have their own comment syntax that you can use, like
{% comment %}
Your comment here
{% endcomment %}
in Jekyll.
I hope that helps!