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