Add integration documentation

[ainmosni]

People who want to join a dataspace using RUN-DSP need to have a guide to follow on how to do so.

This document should be a high level guide on how to use RUN-DSP to join a dataspace, and it should contain:

• A description of what a dataspace is and what joining it entails.
• A small description of RUN-DSP and how it can facilitate the dataspace interaction.
• A small description of RUN-DSP and how it uses providers to interact with the readers infrastructure.
• A link to the GRPC binding repository
• A description of what the functions in the GRPC functions should expect in arguments and what they should return.
• A description of how to test their provider.
• A link to the reference provider as an example of a working provider.
• A word highlighting how RUN-DSP passes through authentication, and how the provider has to handle that.
• A word how the current state is under rapid development and that there's no promise of APIs remaining stable until later.

This documentation is probably best written after #58 has been completed.