Clarify relationship with IETF specification

[FeldrinH]

The readme of this repo says that this is a port of Stefan Goessner's JsonPath implementation. Given that the linked implementation is fairly old and there is now a standards track specification for JsonPath under IETF (https://datatracker.ietf.org/wg/jsonpath/) I think it is important to clarify and document if and to what extent this implementation is compliant with / intends to become compliant with the new IETF specification. Perhaps it is already documented somewhere, in which case I would appreciate if that info was linked to in the project readme.