Document how to write a standalone script

[alandipert]

We don't currently demonstrate the necessity of later::run_now() in a loop for standalone scripting and periodically get questions about how to do it:

• $recv(), deasync or similar #25
• The websocket shuts close without any events emitting #52

We should include a demonstration and explanation of `later::run_now() in our documentation.

[wdkrnls]

I would like to learn how to develop websocket functions in an R package and be able to build up functions using them interactively using devtools commands. Right now document() and test() wreak havoc on websockets and I haven't been able to figure out how to restart them once they've gone dead. While everything works when interactively using the package after it's built, there is a lot of functionality I want to add and the added friction makes things a lot more difficult. I looked at all the packages listed on CRAN with websockets as reverse depends and reverse suggests and only two of them directly test websockets, but both of them only create shortlived websocket objects. The rRofex package uses websockets more like how I want to use them, storing them in an environment, but the package does no tests.