README: Write user guide

[pathob]

The confapi-commons' README.md should be used as a central documentation where we describe what ConfAPI is, how it can be used and how the plugins' index.adoc can be read.

This guide should give an example with an endpoint existing in all plugins (should be /settings), how to get data, how to set data and also explain how the API behaves with unset attributes in the JSON objects

[itWorksIDontKnowWhy]

Done. Waiting for review.

[pathob]

@xHighroad, this task is not done.

• There is no pull request
• There is a branch in the commons project with two commits that don't belong together
• It doesn't look like you checked the resulting formatting of the Markdown you wrote