Custom unit creation not documented

[water-e]

Would it be possible to add a brief example to the Unit documentation page that describes the process of adding units or overriding conversions? We frequently work with practical salinity units and siemens for specific conductance. The former is not there (we think), the latter is. Even if psu were included, we likely would prefer our conversion because it has a tweak for low salinity in brackash agricultural waters. The Unit class itself doesn't seem to onerous to subclass. However it isn't clear exactly all the places we would need to modify in order to either add a new unit or override a conversion if it be necessary (if this is even possible). I see files like site.cfg but I don't know if it will take a second entry or whether I have to add to the xml. I also don't know how to ensure the right imports are made.