Document how to hook in exception monitor (Sentry, Rollbar, etc)

[bensheldon]

The readme should include a section that explains how to set up an exception monitoring tool with GoodJob.

Primary consideration is how to differentiate between

• explicit ActiveJob configuration that allows some kinds of errors to be "expected (unexceptional) exceptions" and ignored/unpreported. For example, an SMTP timeout error.
• implicit ActiveJob configuration, e.g. the root level retry_on StandardError that , if hit, should be reported.

[bensheldon]

Added more recommendations in 27c59df