Improve references to other Specifications in DAPS description

[bellebaum]

The current DAPS README does not reference other specifications, like IETF RFCs, very well.
This has lead to several faulty connector implementations currently blocking updates at the Fraunhofer DAPS.
In order to prevent breaking several connectors, I am updating the DAPS README to make dependencies on other specs more explicit.

Example Problem:

• per RFC 7519:

In the general case, the "aud" value is an array of case-
sensitive strings, each containing a StringOrURI value. In the
special case when the JWT has one audience, the "aud" value MAY be a
single case-sensitive string containing a StringOrURI value.

• From our tests, it seems like some connectors currently only accept a single URI.

[bellebaum]

Someone may want to add the documentation label

[bellebaum]

This has been addressed in the latest updates