1. Create a vNext Class library e.g. EmbedResourceTest
2. Add a resx file
3. Change the namespace in the corresponding designer.cs file to EmbedResourceTest.Core
   note, you need to change here too -

global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("EmbedResourceTest.Core.EmbedMoreResources", typeof(EmbedMoreResources).GetTypeInfo().Assembly);

Try loading the resource, this will fail. If you ignore the namespace and change the above to not include the namespace it works fine.

global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("EmbedResourceTest.EmbedMoreResources", typeof(EmbedMoreResources).GetTypeInfo().Assembly);

This is a very common pattern and it worked in the old world so although there is a workaround it is super painful to keep changing this when you are porting your application to vNext and have large number of resource files (which some projects do).