Add citation and method description for QuEStVar in questvar.py and README

[eneskemalergin]

pyFuncs/questvar.py implements the QuEStVar benchmark metric but does not currently cite the original publication or describe the method at the module level.

Tasks

• Add a module-level docstring to questvar.py referencing the original QuEStVar paper.
• Add a short description of the QuEStVar metric to the README under the Scientific Overview section.
• Add the QuEStVar reference to CITATION.cff under references.