Mention show_exception :after-handler option

error handler blocks won't run in development unless users set that
option

I suggested we deprecate that option in https://github.com/sinatra/sinatra/pull/884 but it's a breaking change for a minor release. So I thought it should be worth mentioning it on README. I want to continue working on that PR to fix another issue where a general `error` block will never run in development, not even if you set the :after_handler option.