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I'll try using GitHub. Right now I've uploaded one mod so far to the Nexus, so I'll probably release both there and GitHub in the future.

I tend to use the mod format suggested on the modding wiki, and include a "description.txt"
 
Yeah, the nuspec format is all about how to define the package structure - it doesn’t dictate what content is included or not. That said, the package format does include fields for project site (this would be the url to the github repos’ releases page or project site), but relevant to your needs the package metadata should contain a short description as well as a link to a more detailed read me (usually hosted at the root of the GH repos).

This reduces the different places with duplicated information and makes one less thing to manage and potentially malfunction up. Build automation can easily make sure that the compiled package includes a description.txt as well as run other checks and such validations on the packages structural integrity
 
Although I definitely see some advantages to GitHub and still strongly recommend it for development, I'm leaning towards Nexus as the semi-official site to release on at this point... primarily because it seems to be familiar to a lot of users and a place that many gamers would more-or-less stumble across mods even if they didn't own the game yet.

I'm inclined to think that visibility is more important than feature set in this context, and the features on Nexus aren't that bad anyway. Any counter-points out there?