Contributing.md says you should link to documentation, so I fixed a few. Still more that need to be done, but I'll send in this for now. 👍

Update name, link to documentation and description of PostNord API. 

Name changed form Sweden to PostNord due to PostNord is a service provider for Sweden and Denmark. 
Description adding Denmark.
Change linked documentation from from specific to general API entry point.

Update the link to the main page instead of the API directly