Add meaningful READMEs

[blootsvoets]

The project code is now only usable by the original developers. Meaningful README's should be added to the root and to each of the modules.

• Root README on purpose, outline of the project, general uses and general guidelines
• Connector README with documentation on how to configure the connector and to implement a converter
• Backend README with configuration options and a link to the API documentation
• Frontend README with configuration options, developer use and production use.

Eventually, publishing information might also be added.