Update VizAlerts install guide to better describe architecture and required settings for ScheduledTriggerView viz

[jdrummey]

See the thread in https://community.tableau.com/message/556231#556231 for full details.

The two main changes are:

1. noting that only allowed_from_address and allowed_recipient_addresses absolutely have to be set for emails to go out.

2. Adding more details on the functions of the config yaml and ScheduledTriggerView viz.

A third change would be to add some more examples of custom code for the ScheduledTriggerView viz.

[mcoles]

I added some stuff for this in the install guide. I'll leave this open for now and let you close if you think the additions were sufficient, Jonathan. I don't believe I added more examples of custom code for the ScheduledTriggerViz.

[mcoles]

https://github.com/tableau/VizAlerts/blob/master/docs/install_guide.md#open-the-vizalertsconfig-workbook

I was going to close this issue out, but I think this aspect really does still need work. No doubt an installation video would go a long way toward helping people understand the difference.