Make readme ready for public usage

[ArneBinder]

Until #183 is implemented, we need at least descriptions for the most relevant parts of PyTorch-IE in the readme to make it usable by the public. This may require the following (entries marked with (!) are mandatory in my opinion).

PyTorch-IE MVP readme TODOs:

• add "Architecture" section (Chris): short intro that says that we have Datasets made of Documents -> Taskmodule -> Model and how these parts interact
• add "Taskmodule" section (Chris)
• cleanup of examples sections (Arne): Improve readme examples #203
• add content for "Development Setup" (Chris)

Note: IMO this is the last missing step before we can start promoting PyTorch-IE in any means.