Add a simple Testing with Rspec section.

A simple shared examples for Rspec that helps people easily add tests to ensure that their models are properly including the `acts_as_paranoid` library and the model's table is properly setup with a `deleted_at` column that is indexed.

_I know you're huge on documentation and your docs for Paranoia are some of the cleanest and understandable I've seen. So, I wanted to include this to help others easily test their use of Paranoia with Rails and Rspec. Also, this might only work for SQL-based databases because it calls `where_sql` and is syntax dependent, but I think it's useful none-the-less._

_Cheers._