Update README for version 2.0 of xnat chart

[markendr]

Describe the bug

README for xnat chart needs to be updated for version 2.0 changes..
charts/blob/main/releases/xnat/README.md

Steps to reproduce

Last README update is 4 Sep 2025

Expected behavior

README updates include new/changed Chart values info and info on impacting changes.

XNAT Chart Version

xnat-2.0.0

Version of Helm

No response

Version of Kubernetes

No response

Additional context

No response