add AsyncPrompt documentation

Docs/asynch-prompts.md

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+# Asynchronous Prompts
+
+In a standard Laravel Prompt, the looping mechanism blocks the main thread while waiting for keypresses. This limits our ability to trigger renders using an `event` based approach.
+
+In `ArtisanBuild/CommunityPrompts/PatchedPrompt` we've extracted these looping mechanisms. This class will not be needed if [this compatibility PR](https://github.com/laravel/prompts/pull/154) can be re-opened and merged.
+
+By extracting these looping mechanisms we're able to overwrite the implementations of these loops in `ArtisanBuild/CommunityPrompts/AsyncPrompt` using a [ReactPHP](https://reactphp.org/) event loop. This unlocks the ability for us to read the terminal, write to the terminal, dispatch http requests, etc, in a non-blocking way. By calling the `render()` method inside `callbacks` we can now do things like debounce http requests to search endpoints, output streamed http responses to the terminal one chunk at a time without blocking the user from entering new text, or even listening for real-time push notifications from a websocket.