Add complete list of all available shipping providers to README

[svenjungnickel]

Hey @engram-design, I added a complete list of all currently available shipping providers to the README file. You might want to add that list to the description in the plugin store as well. I noticed that list is also outdated :)

Btw congrats on the latest release of Postie. You are doing a great job of implementing more features to that plugin :)

[engram-design]

Thanks Sven, champion 🥰