Add in info about inheriting from Sinatra::Application

I am currently teaching 26 people how to program in gSchool. We ran into an issue inheriting from Sinatra::Base where the HTTP method overriding using a hidden form field didn't work when inheriting from Sinatra::Base while it worked for a classic application

This commit adds in a note that you can inherit from Sinatra::Application to get the same settings as a classic application to make it a little more clear.