Add logging example to documentation

Judging by the issues on this package and graphene, a lot of users struggle with graphql-core's decision to catch all exceptions. I see that in #41 something was done to make this easier, but it's not clear how to use it. An example of logging/error handling in the documentation would go a long way.

Start by reading the issue referenced in #41 to understand the existing exception-handling change, then inspect the current documentation for the appropriate location. Add a documentation example showing logging and error handling for graphql-core's caught exceptions, and verify that it explains how to use the behavior in practice.

索引モデルが issue の本文から書いたものです。