Add summary of app structure to README

[KristinaRiemer]

I'm thinking along the lines of the purposes of the most high-level components that make up the app, and then the most important file(s) for each of those components. Initial draft of this below:

1. Cache: to pull down TERRA REF data that will be displayed
   • cache-refresh.R
2. Shiny: to determine how app looks and how data is organized and displayed
   • app.R
   • render-site-map.R (+ other incoming render-*.R)
3. Shiny Server: to host app online
   • shiny-server.conf
4. Docker: to spin up Shiny Server, R environment, and all necessary packages with correct versions
   • Dockerfile
   • docker-compose.yml
   • docker-compose.override.yml

I think this would be useful for developers to get a quicker handle on how the app works overall.