I have a Git repo in ~/.janus/ with a bunch of submodules in it. I want to add a submodule in ~/.janus/snipmate-snippets/snippets/, but when I run git submodule add <git@github.com:...> in the snipmate-snippets directory, I get the following error message:

You need to run this command from the top level of the working tree.

So the question is: How do I add a submodule to the snipmate-snippets directory?

You go into ~/.janus and run:

git submodule add <git@github ...> snipmate-snippets/snippets/

If you need more information about submodules (or git in general) ProGit is pretty useful.