Document Readme allow_domain with more examples

It would be helpful if there were more examples for allow_domain. I.e. how to make an upstream http request and return true or false based on the response text. Another helpful example would be a shell script exit code.

Start with the README's allow_domain documentation and review the existing examples. Add examples covering an upstream HTTP request whose response text determines true or false, plus a shell script example based on its exit code; done means both requested cases are documented clearly.

Written by the indexing model from the issue text.