Clarify STI in Readme.md
Clarify STI in Readme.md

I had to look through the source code to find this. The name of the config item doesn't correspond to the common name "Single Table Inheritance" so it was hard to find.
Sology/sorcery,NoamB/sorcery,rubenmoya/sorcery,Sorcery/sorcery,athix/sorcery,athix/sorcery,athix/sorcery,jshum/sorcery,Sology/sorcery,Sorcery/sorcery,NoamB/sorcery,Sorcery/sorcery,rubenmoya/sorcery,jshum/sorcery