Update Documentation with Best Practices for subclassing an exception

[brock]

This would be really helpful to new users, or users that are migrating from Python2 to Python3.

It would be nice to have an example of a subclassed Exception in the documentation, along with best practices for using the subclassed Exception and how those affect what shows up in Raygun.

After using Raygun for some time, we still find things like unique ID's in the message field, which prevents the Raygun interface from merging these as they come in.

Some helpful posts for us getting our reporting cleaned up are Proper way to declare custom exceptions in modern python and How to make a custom exception class picklable The latter, in particular, is helpful when stack overflow errors are generated while debugging an app that also reports to Raygun.