Document workflow for app development

[cpsievert]

My current understanding is:

1. Add shinyjster::shinyjster_server(input, output, session) to server function (for automatically exiting after shinyjster::run_jster() runs.

2. Add shinyjster::shinyjster_js() to UI add JS-based tests.

3. If this is an app that needs to run manually, add shinycoreci:::manual_app_info$string to the top of the app.

4. Run shinycoreci::use_tests_shinyjster(APP_PATH) to add the appropriate testing scripts

(I don't care where the documentation actually lives, but I think it makes most sense to document in the README in this repo and maybe have pointers from the other repo)