Added basic readme (#5)

README.md

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+# Airlock
+
+![](res/airlock.jpeg)
+
+## Usage
+
+### FFI
+
+Urbit subscriptions require [server-side event (SSE)](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events).
+Unfortunately, Elm does not provide a native implementation.
+This means that we will need to use [JS FFI (aka ports)](https://guide.elm-lang.org/interop/ports.html).
+
+#### Setting up the Elm side
+
+You would need to declare two ports for Airlock to use:
+
+1. `port createEventSource : String -> Cmd msg`
+
+	 This sets up the SSE channel.
+2. `port onEventSourceMessage : (JD.Value -> msg) -> Sub msg`
+	 This recieves the actual messages from the SSE channel.
+
+These two functions need to be passed to one of the functions from `Ur.Run` module. 
+The functions in `Ur.Run` are the same as functions from `Browser`, but have a couple of extra urbit-specific fields.
+
+#### Setting up the JS side
+
+The SSE JS code is neatly packed into [example/fetch-event-source.js](example/fetch-event-source.js). 
+All you need to do is pass them to Elm ports like in [example/script.js](example/script.js).
+
+You can see a full working example in [example](example).
+