Document setting up manual Stripe webbhook testing using Ultrahook

# Problem

One of the features of our API is handling of stripe events via webhooks.

Basically, when one of many stripe events occurs, they hit one of our two endpoints depending on the event type:

```Elixir
scope "/", CodeCorps, host: "api." do
pipe_through [:logging, :stripe_webhooks]

post "/webhooks/stripe/connect", StripeConnectEventsController, :create
post "/webhooks/stripe/platform", StripePlatformEventsController, :create
end
```

The Stripe account dashboard allows sending test events, on test endpoints, but these can't be localhost, so in order to test them in development, we need something like ultrahook, which provides as a quick passthrough subdomain that hits our localhost http://www.ultrahook.com/

We need to write documentation on how to set this up so it hits our proper endpoint locally, if in need of manual testing.