Start documenting TaxJar reporting feature and required configuration

[forkata]

Since we've now introduced some async jobs we need to add some documentation around the reporting feature pertaining to that. Specifically we want to ensure that users of this gem which enable the reporting feature have a job backend configured and if not understand that jobs will be performed inline as per these docs https://guides.rubyonrails.org/v4.2/active_job_basics.html#job-execution.

[Noah-Silvera]

I think we should keep any documentation around how to setup ActiveJob to a minimum. We don't want to start taking on that role, as active job and it's configuration will change over time, including it's current behavior of running jobs inline as there's no default queue adapter.

I think from an extension perspective, we should only inform users that our extension queues jobs with ActiveJob and that they can refer to <link_here> documentation on how to configure ActiveJob

[forkata]

@Noah-Silvera 100% agree, I think we need to document what reporting tasks are performed async and any decisions we've made regarding queue name etc. From a user's point of view I would like to know that the extension uses the ActiveJob API etc. as not all stores use that and some rely directly on interacting with DelayedJob, Sidekiq etc..