You don't need to maintain a list of all your project files in your nuspec file, you can do this:
<files>
<file src="MyProject.Website\**\*.*" target="" />
</files>
This will grab everything in your project (all files and sub directories). If that's too much for you, you can take advantage of exclusion rules:
<files>
<file src="MyProject.Website\**\*.*" target="" exclude="MyProject.Website\docs\admin.txt" />
</files>
Hope that helps.