Update FAQ.md
Update FAQ.md

Extend explanation,
mention reference state
henningjp/CoolProp,henningjp/CoolProp,DANA-Laboratory/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,DANA-Laboratory/CoolProp,CoolProp/CoolProp,CoolProp/CoolProp,henningjp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,DANA-Laboratory/CoolProp,henningjp/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,CoolProp/CoolProp,DANA-Laboratory/CoolProp,JonWel/CoolProp,henningjp/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,henningjp/CoolProp,JonWel/CoolProp,DANA-Laboratory/CoolProp,JonWel/CoolProp,DANA-Laboratory/CoolProp,CoolProp/CoolProp,JonWel/CoolProp,DANA-Laboratory/CoolProp